Is the Jester just completely useless?
Okay, for starters there is dps Jester, and there's tower Jester, like most of the other classes. Both are fairly well OP right now. As others have mentioned Jester towers are not just less expensive than the other classes, several of them even fire faster making them doubly strong. The only limit on them is having to resummon to get exactly what they want, which takes time you don't really get in campaign. Early survival, a dps and the random towers can handle the 1-2 Ogres that may pop out.
For DPS, Jester are top 3 for damage between them Monk and Barbarian. Barbarian can cover multiple duties, including tanking as needed. Monk buffs players and towers around himself. Jester uses any of the most powerful weapons in the game, Apprentice Classic's are good at the moment though any strong weapon is good, while they can have a utility weapon to switch out on the fly like say a 100% block sword. Combine that with top notch speed and health scaling and toss in Fortuna and Move Defenses for whatever the group needs right now.
Fortuna especially, you can heal the whole map's defenses, kill a dozen 20 million health ogres instantly (depending how many are on that map/wave), buff all your allies, debuff all the enemies, etc etc, every 30 seconds. That ability alone is worth a spot on a team, as long as they're geared enough to survive and use it right. It is very controllable, and easy to abort when your first two slots come up wrong just by picking a wrong third thing to do nothing. Worst you might end up doing is slow everything down which is mostly harmless, as that's just 1 slot difference from healing everything.
Ideally I'd want each of a Monk, Jester, Barbarian, and maybe a Summoner for safe uninterrupted repairs for the active group on a late round survival run. I'd settle for just four Jesters though.