Agreed, you can blast folks without this spell.
But if you just want to erase their memory, rewrite it and send them on your way... it's FANTASTIC! Not to mention it wipes out any extended Parma to mundanes so your mates are toast. It's a snap-handy effect and soon to be on my list!
It's also a great non-lethal dueling spell. It lets you get just about anything through on the next shot, and that something could just be embarrassing instead of deadly. This could be good for fights much short of Wizard's War.
Chris
But memory erasing spells don't have to be particularly high level either. If you are competent at memory erasing (and/or maybe have access to Arcane Connections) you could probably Penetrate their Parma anyway.
So, if you are a magus who is competent at memory erasing, then I'm not sure that you really gain much by this strategy (knocking Parma down and then mind-erasing). And for this strategy to work, you need to be competent at PeVi too.
The characters who gain the most benefit by this strategy are those who are much better at PeVi than they are at whatever they want to do once the target's Parma is down. Which certainly means it is a viable strategy for some characters --- but most characters specialised in something offensive other than PeVi are probably more effective just using their specialisation.
That might be RAW for removing hermetic magical effects, but Parma Magica feels like a special case, one which operates outside the normal RAW hermetic guidelines (in fact didn't I read somewhere that it was more like a breakthrough?). I'd be tempted as a story guide to change the calculation against removing parma so that it was harder than the base level mentioned in this thread.
It seems to me that the story would be best served if Parma was sufficiently effective that finding Arcane Connections was necessary, at least for Magi of roughly equal experience.
I think that usually that is the case, at the moment (either that, or good use of Wizard's Communion).
It is true that some specialised magi have high Casting Totals with one small subset of tricks. If those tricks happens to be something that can be used to attack other magi, then the specialised magus can easily trump his contemporaries Parma (+ Form). However, most of these specialised tricks can be easily derailed by an inconvenient Immunity, or Magic Resistance Mastery, or grog, or similar, so they are still not entirely foregone conclusions.
As you say, it is important to consider story too. Whilst it is possible to imagine a magus that is really good in some hypothetical example, it is difficult to see how that is actually a problem in play. To use an example from literature, Sherlock Holmes is preternaturally good at solving mysteries. I don't think that Doyle thought this was a big problem when he was writing stories about Holmes solving mysteries.