Marcinko is an interesting character--I'll leave it at that. That aside, he didn't carry body armor around but then not access it when he needed it; he made the conscious decision (whether good or bad) that he was not going to use it. That's very different from carrying it around in the trunk because you think it might be useful but then not putting it on during a stakeout focused on arresting heavily-armed violent felons.
As it turned out, the vests wouldn't have done much good given that Platt did his work with a rifle. Again, that's a moot point. I'm not speaking to the effectiveness of the vest. The point isn't whether they're effective or not, nor whether there are good reasons for not wearing them.
The point is that if you have essential equipment in the trunk instead of on you when you could reasonably expect that you would need it, you need to work on your mindset.