Yes - I see no harm in it nowadays.
No - There are still risks and problems involved.
Maybe - I'm torn on the issue.
In the past I was against it but Blizzard will likely try to implement pretty much anything as an 'optional' paid service and so at this point I'd say that although I'd have reservations I'd reluctantly accept such a feature. If only because it'd allow me to change my warlock into something I enjoy playing whilst also keeping all the achievements and exclusive items that I earned during the years that I played him.
Maintaining achievements doesn't sound like enough justification to add a redundant service. Just buy a 90 and be done with it.
Also, if you're looking to switch your mains class, you're not as attached to it as you think you are.
To those asking why would you do it, if you already have the boosting service:
There are a lot of people who are very casual raiders who I think would pay for this. Anyone in a strong guild/raid team wouldn't have an issue gearing up alts, but those who lack that support probably would like to skip some of the PITA that geraing can be.
Also legendary quest line... I don't know if I would have paid to skip it on an alt, but when I main swapped at one point... I would have given it some consideration after week getting the bare min on runestones I think it was 3 weeks running, and having to put in a ticket for the one off Sha in SoO, on two of those weeks.
I'm guessing you'll come back with the idea of having all gear in your possession transform into appropriate gear for your new class.
The amount of coding and prep required to do this, plus the sheer amount of possible variations in all the different types of gear in game, is staggering.
How bout.... you guys boost a 90 and get new gear?
I once thought it would be a good idea but now, I find it to be a no as time goes. Don't wish to switch about for such, levelling already too easy.
Stuff can be fixed, just get enough glue or duct tape!
Roses are red, mana is blue. Suramar Guards, Will always find you!
Three expansions in dust; Cata: 1-2 hrs, MoP: 40-60 min, WoD: 60 min (own experience).
Unless the class change comes with a service to convert all your gear, what would be the point?
It's simpler to just level another character.
First of all, as a few others have pointed out as well, boosting to 90 and changing class isn't necessarily the same thing once you scrape beneath the surface. One service's emphasis is to let you skip levelling and jump right into the action, while the other lets you keep your main as your main, with everything that entails (ownership, pride, gear, achievements, name, etc.). The benefit of a paid character boost is that you'll have two characters instead of one, with the drawback that the new one is a tabula rasa, while the benefit of the class change is that you'll get to keep your already built-up seasoned character as a new class, with the drawback that you do not get a second character and everything that entails (professions, etc.).
See? Two clearly distinct services for two very different purposes.
Personally, I have no interest in buying (boosting) a new character of a different class and decking it out. The amount of 'work' (e.g. playtime) involved just getting it up to speed with my older character does not appeal to me. I do have a few alts for professions, but that's about it. I would never go through the legendary quest twice, or forge two Sulfuras hammers, for instance.
I do, however, get the occasional urge to experience the game from another perspective; armed with new spells and abilities, and a different way of playing. At some point, I might fall completely out of love with the dps paladin, and instead wish to play as a warrior tank.
The two options I have is to hope Blizzard makes everything account-bound eventually, or to pay for some sort of class change. Considering they already have support for more or less everything the class change would entail (including replacing gear), I don't see why it's a problem.