I'd be very curious to see what a dev would answer to that question.

Seems like a easy solution, the dual specs system would be technically free too, as long as rumors go, so why bother with it at all?

If it was free, it would definitely make hybrids significantly better and more desired than "pure" classes. Having a cost and/or cooldown associated with it lets Blizzard actually make hybrids competitive with everyone else; if it was free of restrictions, hybrids would need to have significant nerfs to account for the fact that they could do anything, anytime, anywhere, and on the fly.

I think the idea was that they want for you to feel invested in the way that your character is specced. Choosing a spec was meant to provide you with interesting and significant choices that cannot easily be undone.

Fast forward to present day and the cost of respeccing is pretty trivial. Sure, the 50G per adds up over time, but it isn't much a barrier for your typical high-level character compared to bigger gold sinks like nobles decks, mammoth mounts, and fancy rings.

Consider the spells you learn. As a holy pally who is switching to Prot to fill our tanking void, I will lose several talent based spells, and pick up some new ones.

The point is that they will need to consider how to effectively keep the spell abilities, but not allow you to use them unless you are in the right spec. That way you can have different action bars/hotkeys setup for both specs for your talent specific skills.

@Mj

Not really. Haven't you noticed that if you spec, for example, Holy, then went Prot, then back to Holy, you don't have to re-learn all your Holy talent spells? Once you have learned "Holy Shock, Rank X" you only have to learn future ranks when you drop it and re-learn. So the mechanic is already there. The parts that aren't there are: the UI, the how's/when's/how-often's, how to implement switching action bars and glyphs, etc.

I just hope the bar mods are able to adapt quickly.

True.
That would be a simple solution to spec switching.
And I dont think it will unbalance things among pures and hybrids. And moreover the switch is not "on the fly" because you have to go to a trainer in a capital city.

The dual spec system won't give you "Free" specs, the second spec will still have a cost for respec'ing and we all know the cost is there as a money sink. I also think your just assuming that the "Alternate" spec in all cases will be a hybrid spec. That a druid will keep a Dps/Healer dual spec setup. Whilst this might be the case this same druid might well skip between PvP and arena regularly so may well want DPS/Healer pvp specs, in which case he will still want to respec regularly... possibly this is your scenario, hence your desire for free respecs?
Blizz clearly want to keep the money sink but just want to give us a bit more utility to compensate for the disadvantages that Tank/Heal specced classes have in solo content, it's in Blizz's best interest to supply this to folks really as it'll make their instances more frequently visited by people who otherwise wouldn't want to spend the G on a temporary respec... whilst levelling for example.

To my mind all classes have more than two specs they may want at any given time and i will bet you it'll only be a few days after the dual specs are out that people will start asking for "Tri/quad-specs"

Regardless if respeccing were free, Dual specing would be of no interest to me at all anymore, even if it did include glyphs.