I'd keep ballistas for as long as I'd ever wish to compete in iron age PvP tower and use them with a mix of my archers. So this just depends how much you fight for medals and stuff like that. If I wouldn't care about winning lower PvP towers, I'd replace ballistas with catapults(and later on with trebs) as soon as I could.
But I occasionally like fighting for lower age PvP tower crowns so I tend to keep 1 ballista camp around even in LMA.
Your long range shooters(stonethrower, ballista, catapult etc) are always your best anti ranged units weapon, so you generally want to get "rid" of them only if you have the upgrades already done.