From Reddit: NO level sync when entering a dungeon with less that required players!

[Renik]

You would not get roulette reward, because premade parties can't start low level roullete (or how lv17-59 levelup dungeon roulette will be called again). What remains is the exp from the enemies. And look how much exp from a lv 17 enemy you get with lv 50 ^^

It will be more for farming gear for glamour or GC seals. Everyone who want exp will still do roulette.

Last i read about it, current lvl50 and 51-59 dungeons will be part of the high lvl roulette, but the option to enter uncapped in smaller groups will be in the DF, so i guess activating it will prevent the use of roulette, or at least this seems to be the more logical way to manage it.