I used wands all the time with my whm at low levels, mostly so I could use a shield at the same time, but being more experienced now and looking back on it, I probably should have used a staff with the highest +mp I could get, since I'm elvaan. Elvaans have high mnd to begin with, so we don't really need wands, and our defense is high enough to let us take a few hits without dropping into the orange so having a shield isn't that big a deal unless it has some stat bonus you want. (besides, whms shouldn't be getting hit at all in the first place.) As for blm I think I'm going to start using wands, even though I could use the mp...Elvaan int kind of sucks and int is more important to blm than mnd is to whm. For summoner, go staff all the way. Int and mnd are only important depending on what you're subbing, but mp is really a smn's "main" stat.
Now, if you were a taru, I'd probably use wands instead of staves for whm, since tarus have mad mp and low mnd. Same for blm, though their int is higher than their mnd to begin with -- again, their natural mp usually makes up for it. So it kind of depends on what race you are.
Of course, when you hit 51, it all becomes moot -- elemental staves are truly the way to go.