Is there any other reason for the Brigadier slide?
In relation to the race gun photos, yes. The Brigadiers were the first variants to come with factory dovetails for the front sight... an obvious advantage for customization. As well, they feature the beefed up hump on the slide that would withstand higher round count usage. Another obvious advantage for competition.
The Brigadier slide was Beretta's answer to NSW (SEAL) demands for a fix to the early M9 slide separation issue with some Navy guns. Internet immediate argument drills aside, that problem was a reality after the pistols were initially adopted. Not frequent by any means, but it did in fact happen. And not just to Navy guns.
I saw it on occasion (twice) with Army guns during the late 1990s. This with very high use guns in a particular Army SF battalion. Rare, but not unheard of. What was not rare was the routine failure of locking blocks (epidemic in all of my units) and the more occasional (but still common) failure of trigger springs and trigger bar return springs. Eventually leading to later generation radiused locking blocks and various aftermarket upgrades for those two springs.
The fact that Beretta generated two mechanical design changes to mitigate the slide separation issue is a glaring indictment that the problem did exist. The beef-upped Brigadier slide was one fix. Designed to prevent the slide from cracking in half to begin with. The other was to incorporate a modified hammer pin (with an over sized flange on the left end) into all military M9s. Also the defining characteristic of the 92FS. That "S" in the nomenclature indicates the model features that particular M9 fix. That exposed flange (on the left side of the weapon's frame) was/is designed to stop the back half of a broken slide from rearward travel off of the rails in the event of slide failure. Instead, it binds that part of the slide to the frame, keeping it from launching and hitting the shooter in the teeth.
Beretta didn't engineer those changes for a non-existent issue.
I forget how many Brigadier slides were purchased by SOCOM, but in the end, the USN SEAL community adopted SIGs anyway. The majority of those already purchased military Brigadiers slides (gathering dust) got transferred over to Army SF, where they were used mainly by 5th SFG(A) at least through the late 90s and early 00s. The only place I saw them in the flesh was at 5th Group. Other SF Groups may have received some, but I don't have any personal observations of that. IIRC, there were only a coupla thousand purchased for military use. It wasn't a huge number.
I know there's lots of folks out there who've put amazing round counts through Beretta 92 variants. I know Beretta has always claimed very big durability numbers. But the gun was only spec'ed (like the previous 1911A1) for a Cold War requirement of occasional annual firing (a coupla hundred rounds) and a total service life of 5000 rounds. Which translates to about 15-20 years of mild use (mostly stored away in arms rooms). In units that went to high round count routine CQB training in the late 90s... it didn't pan out that way. We broke M9s like beer steins at a Valhalla drinking party. This in units where almost every man was issued an M9. We replaced a lot of worn out guns and parts on a routine basis. Including simply replacing ALL unit guns with brand new ones after just a few years (3-5) of hard use. Rinse and repeat. They just didn't hold up against to our live fire training demands. In a unit where we were ruthless about tracking round counts and proactively replacing things like locking blocks.
In my personal experience (as an 18B Weapons Sergeant, 18Z Team Sergeant, and 18Z Company SGM), the guns simply never lived up to Beretta factory advertised round counts. Maybe they did somewhere else, but not in my Army SF units. At one point, circa 1999, my Company SGM had a little collection of broken M9 locking blocks in a big bowl on his desk. He asked that all ODAs & B-Team guys bring them in as they happened... rather than toss them. After fewer than 10 months, he had 67 of them collected. And that wasn't all of them (some getting tossed anyway). From 83 assigned M9s in the company. That was pretty typical across the board (including all sister companies and other battalions). While locking blocks are mechanically easy to replace, it's more difficult to replace guns with deformed rails caused by the tie-up event... and subsequent wooden malleting (in a shop vise) to get a locked-up gun disassembled. That used to piss me off to no end.
But not nearly as much as having one of my guys with a deadlined weapon in Northern Bosnia... where a CCW M9 was our primary weapon. Further entailing a long drive down to Eagle Base, Tuzla to steal a functional gun from somebody at HQ, and turn the un-repairable one in to depot maintenance channels. And not see it again for months. My guys temporarily out of pocket because they couldn't conduct scheduled work among the Serbian populace without a pistol... and at least two of them gone for an additional entire day (road trip out of sector) to obtain a pistol replacement. Same basic thing during the Kosovo Air Campaign (we were the ground element CSAR package). Same thing in Afghanistan. Same thing in Iraq. Same thing in Africa (except that there was no place to get a replacement pistol). I dunno, dumb grunt that I was, but I seemed to notice a pattern.
I still own a personal 92FS today, 'cause I've got a lot of training & deployed time with the design, as well as a footlocker full of spendy Safariland holsters, mags, light mount adapters, pouches, etc. Sort of a military nostalgia piece for me.
Like my formerly issued 1911A1s, it's an old friend, and comes to hand with deeply learned familiarity. But I don't love it. Because it's the single most problematic handgun I ever used in terms of mechanical parts failure. It's an accurate gun. It's an easy gun to shoot effectively. It's a very safe gun for widespread troop issue. It's a reliably cycling gun as far as digesting a variety of ammo, almost never jamming, and operating well under all field conditions.
But I had enough of them break (small parts failures) in my hand, in the hands of my team mates, and in the hands of most shooters in my units... that I'll never fully trust the M92/M9. Like a partner that's cheated on you, you never fully regain trust. ..even if you stick by them. I never went on a long training deployment, shooting course, or downrange push where somebody's M9 didn't go down hard for maintenance. Usually when we were someplace where repair or timely weapon replacement was extremely problematic. Aggravating, but we learned to live with it. We learned to procure through military channels (or personally purchase) a stockpile of spare parts. And to have a bump plan for shifting working guns to folks that absolutely had to have one. Usually leaving some unfortunate Fobbit without one. Spare pistols carried with us were only very rarely a possibility.
Love/Hate? Yeah... that describes it for me. Although I'm still very tempted to get one of the improved railed models. After my comments above, Go Figure...
Just my $.02 & YMMV.