Not many since we don't have a close equivilent to it (bomb disposal troops maybe).
Which is why you kind of have to take this with a grain of salt. Mil-spec armor isn't for the average, everyday troops. But the heavy infantry that act as a vanguard on a battlefield? Those guys don't exist in real life. There's no equivalent to them at all. There's no way to determine who in the real world would use that kind of gear, simply because it doesn't exist. As to the whole "Red Samurai" thing - those are the elite military of Renraku. They're exactly the kinds of soldiers I'm referring to as "heavy infantry." And the Tir Ghosts? Those are very literally a heavy military unit. Sure they have light forces as well (who are arguably better than the heavy guys) but they're also exactly who I'm referring to as "heavy infantry."
Maybe I'll change the terms I'm using.
Scouts and other light forces would use armor vests, maybe jackets. The exception to this are highly-trained Chimera, Tir Ghosts, etc. who probably use specialized equipment like mil-spec armor.
Medium infantry are the bulk of any military infantry. These guys use jackets, and rarely anything lighter than this. They might use some light mil-spec gear, but it's unlikely.
Specialized units are the cream of the crop - these are the Delta Force guys, Red Samurai, etc. These guys use mil-spec gear when they're rolling heavy, but will use anything that's appropriate to their environment and opposition.