To summarize what people are saying:

Early on: Raise them, level 5 manyus only (maybe 20s, don't bother giving them 100s), they're necessary to fill out teams.

Later on: They'll get phased out as you get event 5*s and the occasional lucky gacha pull. Thus the reason you shouldn't invest too heavily in them.

If you've been playing for months: Only really good for, uh... "research material"