Cailleach Magic (Hedge Magic, page 56) already works like a major focus for spontaneous. And since it's a Major Supernatural (ie, non-Hermetic) Virtue, it's perfect for a non-Diedne spont heavy ex misc.
IMO, Major Magical Focus in spontaneous magic doesn't satisfy the test of being In a certain limited area. Before someone counters with the Tremere House virtue, I think that also fails, but can be easily be called Certamen Focus and let the mechanics stand. This also lets Tremere pick magical focus...
I am in full agreement with you. I think spontaneous magic is far too large an area for either Potent Magic or Major Magical Focus. I was just playing off the opening post where Diedre magic was broken into two different virtues one one of which was Focus Spont Magic.
Besides, what's the point of being a munchkin is you cannot take a house rule on a minor issue and abuse it.
While we are the subject I have never liked the Tremere House Virtue. Just my 2cents worth.
In some ways, I don't think that there needs to be a spontaneous magic virtue that operates like the Diedne virtue. Mercurian does assist with Ritual magic, but how often are you flinging about rituals in a saga? And Method Caster applies to both Formulaic and Ritual spells. If you're pursuing spontaneous magic, you likely have a generalist magician. In my experience, these are not so much fun to play.
From an in-play perspective someone with a spontaneous magic virtue needs to have encyclopedic knowledge of all the spell guidelines, and I do mean all, so that he knows what he can cast at any given time. Someone who is specialized in 1 or 2 Techniques and 1 or 2 forms has only 4 sets of guidelines to keep track of closely, and a few more within his specialized forms to know of in an off-handed fashion, and the other stuff he lets slide to his sodales. This is why I think specialized magi are much more interesting and a much more fun character to play, IMO. Specialized magi try and solve all problems through the lens of their specialties.
 My only experience in playing with a Diedne magus was shortlived. This was in Bibracte and to wrap up his character creation process (which is 30 year PG advancement with 40xp per year, buying spells with XPs) he ended up spending his remaining XP in his Arts bringing them all to a scores around 10 +/- 1 or 2. He ended up with a very generalized character who could cast any 15th level spell. That seems powerful, but keep in mind that the player has to know every 15th level spell, which goes to my point about knowledge of the guidelines.
personally remember in "frightening munchkinism" a beautifull concept of a ritual spont caster and i liked so much
regarding the spells remembering i would disagree, after played mage ascencion i understand spont magic in a different way, forget spells and go for guidelines. you would be able to apply up to 2 magnitudes... play with that.
hehe thats said by one last character is focused in 1 art XD