Hey! Yes, it was the intended design that most casters wouldn't risk much going beyond 1~2 [DICE]. We talk about it on Spellcasters NPC's in the GMH:
"Truly legendary Spellcasters might go above that or even have an unlimited Power Budget. Be mindful that they are still susceptible to Mishaps, so even careless NPCs will keep most castings to 1 or 2 Power, only going above if truly in need."
The deal with 3 to 5 Power was to emulate Spellburn in DCC, when the caster needs guarantee higher effects in a life-or-death situation. But I don't like to take it lightly as in DCC where Spellburn could have no effects so ever. You're breaking the laws of nature, deal with it.
Spells are the only way to break the max 12 Damage a target can receive per Round, I used it a few times like this, obviously taking a Mishap.
So yes, 1 Power should be the common ground, 2 kinda risky and beyond that you're on your luck.
If you want to allow higher Power in your games, I would count only double 6s or remove Mishaps completely and have the caster receive [SUM] Damage when they get the same number in two or more [DICE].