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"

It just depends on what do you want to do in that game. On my side I keep all Knights, including all 2* and I have even some of them Evolved and maxed. I have also a Squad with Mint 2* who farm the most dangerous maps.

1. Collect them all and don't keep copies.
2. Use the copies to upgrade their skills and equip slot.

They're easy to collect anyway and you'll max everything except their level since at this point you will be focusing on 4star or 5star girl.

If you're looking for a weakish raid boss squad and use the skip button for fights...greens and unevolved bronzes seem to get through fights faster because there are none of the ability activation messages.