As well as the starting 17 most SL clubs carry an extra 10-15 players in their squad. Tangata and most of the other better Championship players might not be starting 17-calibre in SL, but they'd be at least as good as some of the players at the bottom end of the roster. In fact, large numbers of Championship players were once reserves at SL clubs and most of them don't prove dominant at Championship level.
So often, it depends on the player's circumstances ie does he have a good job outside rugby and want to stay part time, or is he itching for a crack at playing full time?