Back in the day, always trained with either the kit we used or if in an overtly civvy environment, a suitable non military style loaded pack etc, makes sense for all the tactical reasons.
BUT fast forward thirty years & my body is borderline destroyed, every kind of joint & arthritis related problem, but I don't think it was the wrong way to train, I still think if your "work" is of a military or police nature then your fitness regime needs to reflect your "on the job shit hitting the fan" needs.
Be interesting to know if the op, or anybody else witnessing the use of plate carriers in the gym, was to ask if it was job related or just a new gym bunny fad.
On a possibly related side note, a friend, a former officer in my old regiment who's now involved heavily in worldwide pmc/security contracts, was telling me only this week that the mod is "restructuring" all of the fitness tests, at both intake level & recurrent cft's etc, because fitness levels have been so poor.
The recurrent testing was running almost 10% failure rate, & recruit level was much higher.
Under the new "restructured" system, it will be almost impossible to fail......... wtf is that about ?