Well mostly it's that it's a waste of time.
Like yesterday I was in a party with friends who were 1-3 while I was 5. Because we had a level 1, we went after lvl 3-4 monsters first.
Thing is, attacking lower level monsters = no chain. So while some of them (level 1 and 2) were getting chains, a level 5 like me gets no chain. So it's little experience, no crystals. I was lucky that a NM spawned, because that actually does give something and it leveled our lowest person faster.
I didn't mind it because I knew he'd catch up eventually and once he got a second magicite we'd go a bit higher, but that's when I was only initially 4 levels higher.
If I could still get some crystals and little to no exp helping lower level people, that'd be nice.
If I were level 17 and a friend/FC mate was just starting out at level 1, that'd be a whole lotta hours of no rewards helping them.