^ What they said.
The thing is, with the three class system, you can have your cake and eat it too. If you are looking for a third class, you need to look long and hard. If you're looking for a second class, you can pretty much pick whatever appeals.
About the only bad mage combo is m/r, and that's just because the costs outweigh the benefits- it isn't a complete write-off by any means. M/Wd and M/S are both great for something- m/wd is potentially the highest performing PvP mage. M/S is a very well rounded PvE mage at endgame.
If you don't yet have a second class, my advice is to go healer just to make your life easier. Leveling is much more fun when you can heal yourself. If you did pick a healer, then you can choose a tertiary to complement the healer you picked. For example, if you picked m/d (a fabulous PvP mage) you could do m/w or m/s for PvE. If you went m/p (a great PvE mage) you could pick m/wd or m/k for PvP (or you could do like I did and pick m/s and use p/s for PvP

). Or, you could throw that all out the window and pick basically any random classes, because mage/anything is good for something.
M/P- good at everything. Perfect solo class. Good in PvP, good in PvE.
M/K- great in PvP, useful in some very specific PvE circumstances.
M/D- great in PvP, easy to level, substandard in PvE.
M/S- so-so in PvP, fabulous at PvE.
M/W- next to useless in PvP, terrific at PvE.
M/Wd- terrific at PvP, substandard in PvE.
M/R- OK at PvP, OK at PvE- ruined by bad dual wielding weapon options.
Pick two that appeal, and you'll be good to go. Don't be afraid to give m/wd a try, or m/s for that matter. I love love love my m/s, and a friend of mine rocks the m/wd in PvP; they are both good combos and fun as hell to play.