I would personally prefer to go healer, and keep the warr up while he chain pulls and kills half the world. If the warr keeps a shield in the backpack, you have a super easy time putting together a group for any instance you'd like to visit, and a lot of the time, do not even need to look for more in order to do normal group quests. One more bonus from being a healer is that you can make do with bad gear (nothing will hit you if the warr handles his job). Quest greens loaded with +healing/int/spirit and maybe a boss drop or two are plenty. It saves you that much more time and gold, which are better spent leveling higher and buying mounts.
Once you hit high levels, you might as well try other specs. It's cheap while leveling as there is not a lot of challenge and you won't have invested much in gear yet. Going retri at some point in 60-70 would not be a bad idea. I still would not go prot as long as you are with the warrior.
From 70+, start considering what you want to do at 80, and take suitable instance drops/quest loot for that spec.