I agree with you when it comes to Levy Pikemen. If left on guard mode to stand there, they can usually hold their own for a bit, and are better at blocking streets during siege battles. That said, I'm sure phalanx formations break up much easier than they used to, so they're pretty hopeless in hand to hand, worse than the hoplites.

I can't remember, but I think the upkeep per unit works out the same, so you might as well have a 240 man unit (based on huge setting) than a 160 man unit for garrison duties.