I mean....Jon pretty much was/is Clark's sidekick, regardless. Every definition of sidekick pretty much fits the bill for this particular Superboy. And there's nothing wrong with that. Even when he's older, he's a kid who's learning from his dad. Of course that's the role he'll default to while they're working together.
But that aside, treating the Titans as just a gettogether of sidekicks does an extreme disservice to what they've been since, like, the 80s (And not even that accurate of an assertion considering the most well known combination of Titans only included one "sidekick"). Yes half the roster still tends to be legacy characters but those books are also where a lot of those characters do end up growing, outside the shadow of the heroes that mentored them.
One of the best examples I can think of, outside of the original five, is Cassie. Aside from a very...disturbing roadblock when Conner died, the amount of growth that she went through during her time as a Titan (pre-New 52) was what made her my favorite Wonder Girl. Whether it was her growth into the team's leader, her subplot with Ares, or her friendship with Supergirl; Cassie (and, to a lesser extent, her Young Justice teammates) really grew up and came into her own when she became a Titan. Because that's what that team is supposed to be about. Young heroes growing and coming into their own in an environment away from their mentors (if they have any. Again, only like half of the more well known Titans are "sidekicks"). If growing up under the Titans had even a small chance at yielding the same kind of growth for Jon that it did for characters like Cassie, the NTT, or....honestly, even Damian's second team, then I'd want him there in a heartbeat.
The only legacy characters who (arguably) didn't benefit from any of his time as a Titan was Damian. Which is a shame because I actually had high hopes for him on that team. But that's a totally different rant.