Hmm...
Okay, there are usually only a few reasons you keep cards in your hand.
A) Dead draw. It is a weapon or shield and you already have one on the field. Nightmare is also a dead card.
B) Unfulfilled Combo. Example: You have a key card and are waiting for quint. This card is strongest in this scenario. However, looking at it from a different perspective, the most popular decks are both rainbows and cremation decks anyways, and I feel they could use the slight nerf from a risk of getting their combos disrupted.
C) Not enough quanta. This happens in the earlier game, but once you're at QOP this doesn't happen anymore.
D) Conditions not met. Control without a target, or Healing at high HP. You'll hang on to those cards until you need them.
E) Part of a chain. You're chaining cards and you have one currently on the field.
F) Saving for the OTK. This card is also powerful in this circumstance.
If you don't fulfill these conditions then you're in top-deck mode anyways and this card is worthless.