The problem with that is without being able to go passive in combat makes it very difficult to get out of a battle regimen if you put up the wrong attack or ws, for instance. I'm sure there are other tactics that involve going into passive mode as well. Even if you need to run away you're faster by putting your weapon away. Being able to put your weapon away allows for a lot of freedom and outside the box thinking.
I honestly feel if you think that the mp regen is making it 'too easy' for your mages, then I would like to contest how good those mages are. MP = power for a mage. Maybe I'm biased because I never do parties over 8 people (if I even get 8 people.) We always keep the same two mages during NMs, myself and one other, and we are nonstop magic/healing/debuffing as a mage should if they have that mp. If someone gets low on mp, we will put our weapon away, but usually only to run off and grab a mob for us to siphon off of. If you just stand around waiting for mp to go up from the mp regen you are just wasting time. Tranquility/Exhaltation/Siphon MP are much faster and you are a better asset to your party for it.
I will say that I was never for the mp regen because I felt this sort of thing will happen. Mages that think they are only there to cure. Putting their weapons away and only waiting for those who need heals. I don't care for this combat style and find it lazy on their end. A mage does more than just heal. However, I am also anti 'nuking' mages too. Some mages on my server refuse to even equip a heal on their bar and they are nuking for like 50 damage on an NM. It's wasteful and it just lets down party members. Parties are a team effort and no one else is allowed to just put their weapon away and wait around like that. It shouldn't be that way for mages either. MP regen just gives people an excuse to, but it's those players choices, not the game, that causes that to happen. Before and after MP regen, as a mage, my combat style and my choices have barely altered. I hardly even consider the MP regen because that's how much it matters to me.