But it's not a slippery slope argument OR a false analogy argument. It's called reductio ad absurdum, or "reduction to the absurd", and it is a very common form of argument. It's also very effective - it's the entire basis behind Stephen Colbert's brand of comedy, or The Onion's brand of comedy (or at least, it used to be). Look it up sometime.
But it shouldn't even apply here because the mere notion of switching ACs is horribly absurd in itself. Say you wanted to switch from a Guardian to a Sentinel. Your character would literally become stark naked in the middle of the Republic Fleet, as all your heavy armor is now useless on you. Now say you wanted to switch from a Sniper to an Operative for the heals. On top of reconfiguring, ooh, essentially your entire interface to adjust from cover-based ranged combat to melee combat, stealth and healing, you realize that you have absolutely no useful measure of alacrity, something that an operative/scoundrel needs to heal remotely effectively (not even healing, just energy management at that point with the increased speed of Diagnostic Scan). Oh, and your main weapon is now junk, since Ops use Blaster Rifles and not Sniper Rifles. That's just gear limitations. Never mind that you haven't had the countless hours of preparation that made you slightly more effective than the guy across from you, who also had countless hours of preparation on him. You're asking to be turned to a baby in a man's body, because you're switching from one set of tools to another. It's the difference between driving a drag racer and a rally car.
The sad part is, every class has a shared skill tree with their other Advanced Class, not a perfect match because of core mechanics, but really close enough, to ensure that people can still experience that hybrid of sorts. It's also quite useful for PvP.
If Mercs are terrible at PvP at the moment, well, that's the problem Bioware needs to solve, and I would wholeheartedly get behind that. But switching ACs is simply absurd. I had trouble switching between specs on my Sent (Combat to Watchman, and I really do enjoy both trees, but I did need a bit of time to get used to using the other half of my Sentinel skills), I couldn't imagine switching from a Sentinel to a Guardian. Odds are, by the time you learn your new class (which, by the way, have their own problems too), your original class will have been buffed. And you will be stuck holding the pieces.
Oh, and you'll have to pick up all the new scoundrels/guardians/ops/jugs who switched from their pure damage classes just to get into Group Finder queues faster.