No. There are two main benefits to armies, as far as I can tell.
- Their hitpoints are massed. So imagine you had an army of vet cavs (12 hp) attacking a rifleman in a city on a hill. You might lose 10 of those 12 hp, equivalent to at least 2 dead cavs if you'd attacked separately (and maybe more, since the rifleman would've had a chance to promote), but the army would still be alive, and as good as new after healing.
- Armies can blitz (i.e. attack more than once per turn). (I think this is true for all armies, not just multiple-move armies - someone correct me if I'm wrong.) So if you're going up after weak defenders, you can still take more than one of them out on any given turn.
There's also an indirect bonus in that the AI really doesn't like to attack armies, especially defensive armies. If you need units to protect that stack of 50 artillery you're using, a defensive army is the way to go.
If you have an army of like units (i.e. 3 swordsmen, 3 cavs, or 3 infantry), the attack, defense and moves will be the same as for an individual one of those units. If you mix fast and slow units (i.e. swordsmen and a cav), the army will move as fast as the slowest unit.
If you mix units of different attack and defense values, I believe the initial attack/defense will be by the strongest unit, but if you lose (in that same attack) all of the hitpoints corresponding to that strongest unit, the next strongest unit's attack/defense strength will take over. On the next turn, though (or possibly even the next attack in a blitz - I'm not certain), your strongest unit will be fighting again, even if the army hasn't had time to heal.
Clear as mud, right?
Renata