Decisions about how to make a TV adaptation of an earlier concept shouldn't be based on copying what was done before. It needs to be treated as a distinct work in its own right with decisions based on what works best for it. And I think, looking at it from that perspective, that using time travel too much would've overcomplicated things. It's also rather unsatisfying dramatically if the whole thing is circular, if Clark ends up deciding to go a certain way because someone from the future told him he was supposed to go a certain way. Frankly I think that's a ghastly idea. I'd rather see Clark making
himself the man he'll become than just being told
who he'll become. The fewer advantages a character has, the harder it is for him to achieve it, then the more worthy his achievement becomes. If he's got future fanboys holding his hand and guiding him, that makes it less of a personal achievement than if he find the answers as he goes.
It's also way too limiting. Even with all the modern fondness for long-range story arcs, a show still needs to have the flexibility to change direction, to adapt to new ideas and real-world problems. Look at how the show's had to adjust to Michael Rosenbaum's departure and now Allison Mack's greatly reduced presence. It would just be a bad idea to include characters from the future on a regular basis, because that would lay down too many data points about the future and make it harder to change if the need arose.
That said, I would've loved to see more of this show's version of Imra, because she was gorgeous and an effective character. I have nothing against occasional Legion appearances. But I don't see any good reason why it should've been instead of
the present-day heroes.