actually, i was aiming for that with mei (who still needs both skill rate and equip slots increased), but considering i didn't have 2 of the 3 rainbow girls.... i got the 2 i didn't have as luck would have it
even then, i knew of a few gold units i didn't have at 4 equip slots yet and then on top of that I easily could use more rainbow dressblooms. most of mine have 1-2 slots cause i have so many rainbows now (21 in fact). Only Lycoris had 4 slots cause well... she's top waifu for this game (in my case).
value is honestly subjective, but i can still agree with what you said, the rainbow medals would greatly help (which is why they need to implement the damn leader gacha already....)