Battles may not have been decided by seige style weapons back in the day but they are a real morale killer, nobody likes watching the 3 blokes next to them squewered on a giant arrow, its a bit of a downer. The idea of roman field weapons was usually to make the enemy charge the romans. Not many armies are willing to stand on a hill all day while the enemy chucks giant rocks and their mates assorted bodyparts at them from beyond bowrange.
Also marlowe, real life isnt battletech, armour doesnt stop things or let them through, you still get hit and those small wounds add up when trying to some bastard is trying to cave your skull in. A nice big shield with 3 javelins in it is also more of a hinderence than a help when said bastard turns up.
Trying to take out the heavy stuff with skirmisers is fine but you have to do something about the 500 mean looking buggers in tin hats standing just next to them first. Most of this stuff had wheels and could be set up fairly quickly, the romans even had special man portable ballistae for mountain fighting.
All of which is pretty much right, however you've basically pointed out that the real point of these things is not to win battles by themselves, but to deny the enemy the option of being static in turn, and that it's the 500 mean looking guys in tin hats that will decide things. Ultimately, people are not going there and all conveniently get shot, they're going to do something to stop it, and you need to have a plan A.
As for being "reasonably mobile", they're still the slowest thing on the battlefield, their ammunition is bulky and needs to be carried as well, setting them up close enough to be effect is STILL going to be pretty hairy, and you're probably going to want to entrench them as well, since they're not going to stop a Sarmatian or Sassanid or Palmyran charge by themselves.
Interesting you mentioned mountain fighting. As it happens the latter war against the Dacians in the Carpathian mountains IS the major appearance of things things in history. I don't believe they saw much use in other times, if they even existed in any other time.