Superboy's ability to travel forward in time must have required a huge amount of self-control not to misuse. He could learn how his foster parents or friends would die, how he himself would die. That's knowledge mortal minds are not built to handle.
And to the Legionnaires, Superboy would be a bit spooky to have around. It would be like hanging out with Grandpa as a kid. I know these Legion guys are used to strange situations but still. Imagine having (for example) Jack Kennedy showing up every now and then to give advice, not knowing his own fate, and then saying goodbye with a cheerful, "Well, I'm supposed to meet Jackie in Dallas today, so long." And you couldn't say anything.