Christopher has a valid point about the budget for the series being astronomical, also about a viewing need for a show like the Legion. It could be one of those series with a niche following but not much else, some fans would argue that's exactly what Smallville has become in the last few years. The fact though is that Clark's ties to the Legion can not be disputable and I really do think they should have appeared a lot sooner than they did.
With regards to the Justice League/DCU characters appearing. I like the solo episodes where they appeared and I thought "Justice" was just a spectacular episode but ultimately I disagreed with having them on the show before Clark was Superman.
The thing about "Smallville" is that it is supposed to be about Clark Kent's formulative years except in the way the series has been presented, they've exhausted that part and now are telling stories about his early adult life, while still holding strong to their old formulaic style and of course reinforcing with an iron fist "No Flights, No Tights".