There are several things veteran players consider about a card right now:
1) performance
2) trading value
3) potentials
Performance-wise, R. Banshee is a weak card. Her best skill Revive doesn't always happen and she cannot one-hit-KO (OHKO) many guardian. You need to feed her a skill+4 stone to replace her current Dark+3 skill. Her stats are spread evenly across the board, which doesn't make her anything unique for a typical invitation card. Due to her higher WIS than ATK stat, her strongest attack is magic elemental, unlike Iron Maiden who is a physical atacker.
Trading value seems higher these days due to lots of people having her card, and it seems the easiest card to pair with, mainly because there is only one type COOL, and a lot of people have them. Still, her trading almost never went more than 20 stat stones (iii), although I did got a COOL Gilgamesh fsemp for her once. When it comes to an open market, it depends on what people are looking for. If someone is desperately looking for a Requiem Banshee and you have one, he might be willing to trade you with some good deals, but for everyone else, they won't even want to buy that card even for just 10 stat stone (iii).
Potentials are good for her coming the Rebirth system. But she'll only be a stronger 'weak' card. Good for Novice Coliseum and Master Coliseum up to Rank B. Almost nobody uses her for Elite Coliseum except for this week when 'ladies only' guardians are allowed.
CONCLUSION: She is the weakest card among Invitational Cards but the cutest one. Thanks to her name
Banshee, she is in high demand strictly for collection purposes by
pedoplayers who loves Banshee collections. I'd use her purely as trading bullets to trade with other better cards especially when she is in high demand. If you're a newbie, she is more than enough to help you with the normal quests and submission battles.

looool.
So I guess i'll still level up Requiem Banshee mainly for quests?
"Strong" cards are those with high ATK that is able to OHKO opponents? Iron Maiden > Requiem Banshee?
yes IM is faster and more lethal. but sure, level up r.banshee to use and you can sell it off later.