I'm in two minds on this one. If Mark can drag Linux kicking and screaming past its current desktop boundaries into popular relevance, that will most likely benefit it as a whole. He's interested in making it popular, and whether or not you like Unity, it's shiny, and shiny appeals to the end user. (It's a reality I despise, but Apple's kinda proven once and for all that end users are magpies.) Besides, just because newbies use a Linux distro isn't going to suddenly turn the kernel into Windows ME.
On the other hand, Linux bods have long since resented moves that are overtly commercial, and if he's going to continue to make Ubuntu popular, he needs the support of Linux's broad base. Move the system too far from that base, or piss people off too much, and he risks losing contributions.
It's difficult, but I'm continuing to support him thus far. Unity isn't the horrendous system-breaker people pretend it is, no more than Metro is on Windows 8 - those are just knee jerk reactions, and I'm guilty of them too. The plan's ambitious, and he has a Molyneux-style habit of talking up shit he hasn't built yet, but if he can pull it off it'll be bloody impressive. Mithras knows, I'm begging for a proper Linux OS on a proper smartphone, like I used to have before Nokia went batty nuggets.
Remember, no matter what vitriol you can hurl at his UI, at its core the distro is still very good. When Ubuntu has done things differently in the past, it's generally worked pretty well. I still run Kubuntu on everything I own - the solid Ubuntu base with the KDE I like over the top. (I did this long before Unity arrived.) No matter what happens at this point, it'd be impossible for him to take that solid base away from us.