I'll take a crack at explaining it's usefulness. First off, proing it out should be your first step. With the the perk you can change your class anytime in five seconds. Pro does it in three. I had (Prestiged level 3 just now) my Sniper (Stopping Power, Steady Aim), Rifleman (Hardline, Steady Aim), Grenadier (Danger Close, Ninja) and Stealth (Cold-blooded, Ninja) Classes all with One Man Army. The use varries, but let me throw a few examples.
Example, you're on the rifle class, all the sudden air support such as a chopper jumps in. Rather than hide all day, you can pull up One Man Army, switch to stealth and hide in plain sight. Chopper goes away, switch back to your rifleman. Say you want a spotting position, but there are dudes possibly in the way. Use Rifle, Grenade or Stealth to barge your way in, then pull out your sniper rifle and get busy.
Changing classes on the fly is the major advantage. While you do loose out on a secondary weapon, there aren't a whole ton of times you need it. Even if you do, you can still have a class with a secondary available, you just cannot change back.
Finaly is the major advantage to it. You can change classes to the one you're currently on and replenish ammo and grenades (and your wepon if you threw it away). Here's what I do to be a cheap bastard. Camp with the grenadier class (M203+Semtex), fire off with reckless abandon, One Man Army to itself, fire off another three, rinse, repeat. This works way better than scavenger for the grenadier because by the time you have replenished the M203 with a blue bag you can use one man army to get two more grenades.
You can call me a n00b, a cheap camper or an explosives whore, but I live by the philosiphy of Price/Shepard. "Are you prepared to do whatever it takes to win?"
