what
I wonder if there are two different arguments about downtime going on here.
The one that I think of is from the perspective of raiding. I haven't raided in a long time, but I can remember how tedious it was. It's not that it took a long time--it did, but that was okay. Rather, it's that so much of that time was spent doing nothing. Can't do anything when there's someone with less than full mana, so you wait to med. Someone needs buffs, so there's another 3 minutes making sure they get the right things and another 2 to med up again. A healer died, so now we're waiting 5 minutes for them to get rezzed and DF to wear off, then another 5 or 6 minutes for them to med up. And if you wipe, well, you're not gonna be doing anything for the next 15 to 20 minutes. Waiting, waiting, people could be ready any second, want to get this over with as soon as possible, but of course even when everyone is buffed up and FM someone is inevitably afk. And you know what? They're perfectly justified in being afk. For the last 15 minutes, none of you have really been playing a game at all. Might as well actually do something with that time.
I support shortening downtime from this point of view; hell, the time I'm harkening back to didn't have medding during DF or reduced buff mana costs in raids. Meditate was already sped up once before back in the day, as well. But when it comes down to it, there isn't really anything more challenging or engaging in being forced to wait around doing nothing for 5 to 20 minutes than there would be in being forced to wait, say, 2 to 5 minutes instead. It's really time spent not playing the game.
From the perspective of exping and whatnot I guess it would be different since you aren't necessarily going to med to full between each fight. But, if I changed meditate I'd probably just make it set mana to full if you are out of combat and meditating for two minutes straight. There wouldn't be any increased mana regen before then. If someone in your group is pulling that would probably disrupt it; and in any case, if you're medding for two minutes straight in an exp group with pre-raid gear you probably would have ended up FM anyway.