Not an expert on this, but here's what I'm seeing:
-Not everyone wants to subject their products to NIJ testing, whether's it due to the expense, or lack of perceived need for it
-NIJ testing for 3/4 doesn't mean all that much, it's not especially relevant to today's threats.
-Ratings from the manufacturer or vendor may or may not mean much, the devil is in the details. You have to inquire about the details of the testing. For example, just because a plate is claimed to stop M855 doesn't mean it will stop M193. Just because it'll stop M193 out of a 14.5" barrel doesn't mean it will stop M193 out of a 20" barrel.
Very valid points. It is always a good idea to request the test results for your plates of interest. It seems the current NIJ standards are more a means of decreasing liability on departments and mfgs and a way of ensuring funding (ie: grants) than an efficient operational measure.
"Special Rifle Threat" validation is the current trend which means you have to even more carefully consider what threats a plate is tested to stop and which threats it is likely to stop. The OPs point is probably the most important about body armor, determine your likely threats, weight, and cost requirements then narrow down from there. As implied, body armor is a confusing topic.