I'd love a dual spec as well. My priest has 20 days played and my respec costs are already 50g.
I basically have to respec every weekend to make some gold to pay my potions for the week, to grind a bit for my epic mount, grind some primals for cloth, etc etc. Being a mage was easier

Well, it's been popping up on the suggestion forums 10-15 times each week at least as long as I've been playing WoW (Mar 05). I think it's safe to say we won't be seeing it.

50g is the new 10g, I don't think respeccing is at all prohibitive anymore; this is pretty much a non-issue.

Highly dependent on your class, play style, play time, ability to farm, and guild, Dozer. Not to mention, the gravy train that is quest gold will run out eventually. You cannot rely indefinitely on knocking out a couple quests to provide extra gold income. I know a few warriors in my guild who sink at least 100g in respecs per week, on top of farming raid consumables and doing Arena.

A WoW world without money sinks would probably be very dire and unpleasant. As much as we say we would love and benefit from free respecs, we'd probably feel the pinch in other ways. I'm sure if blizzard dropped annoyances like durability and respecs we'd all be paying 500g for a stack of silk cloth in a years time, and the game would become absolutely prohibitive to any new person trying to start.

I think a dual spec system would be much appreciated by everyone, not just dual role classes. Ideally it would be a system where you could set up your 2 specs templates at the class trainer for a flat fee (maybe 100 gold, nothing horribly expensive but enough so you have to think carefully about it). You could then switch between specs at the trainer 1 time a day for no additional cost. Once the templates are set up, you shouldn't even have to rebuy spec dependent spells every time you switch, you'd just get them with the spec.

Does anybody have a counterargument as to how such a system could be bad?

can't do that if you're *the* MT that has gear stacked on him, unfortunately. (meaning the other warriors have considerably less gear.)

(edit: i respec 2-4 times a week for pvp, and now that I finally got my epic mount, I just farm by mining. Then again, that ups my gold/week farming requirement quite alot due to the arena respecs :/ I also buy my own flasks/stoneshields etc.)