I've upvoted you because it's probably true...it probably is a vocal minority (Linux users are very vocal and a minority).
But, I also think it would increase the appeal of the platform for a lot of developers. Developers love Linux at a rate far beyond that of the population at large. I see maybe 20-40% Linux users at even non-OSS oriented gatherings of hackers. I do all of my hacking on Linux, and wouldn't have any interest in working on any other platform for development (even Mac OS X pales in comparison, for me, due to the oddity of its UNIX environment, and the inflexibility of its windowing system; also lack of middle-mouse button paste is a minor but frequent impediment to productivity for me). So, while I've had a few game ideas over the past few years, and would like to have a nice, fun, environment to work in for building them...I'd never take the plunge if it didn't work under Linux, because developing without Linux is like a day without sunshine.
You also have to realize the availability of a webplayer and the complete IDE for linux are different things. What that vocal minority seemed to aim for was the ability to play the unity games and not to have the IDE for linux.
Unfortunately, realtime 3D graphics programming on Linux is something of a sisyphean task. Even the best drivers (the proprietary nvidia ones) are somewhat buggy; the intel ones and both the open source and the proprietary AMD ones are even worse. That's if you can even get them working with your hardware and distro. I say this as a long-time Linux user and graphics programmer. It's pretty sad, really.
I agree on all counts. I have machines using all three graphics drivers, and the state of play, so to speak, is abysmal. Linux can't even play full screen HD Hulu on an ATI 4850 video card without stuttering. While we're on the subject, audio is in just as sad a state. I've been trying to use Linux for prosumer level audio recording for a dozen years, about once a year, and every single time I've gone wimpering back to Windows for audio work. So, yeah, the state of multimedia on Linux, while dramatically better than its ever been, is still embarrassingly bad.
So, yeah, I probably wouldn't want to be the developer tasked with making Unity work on Linux (even a very limited subset, like "just nVidia and ATI provided drivers"), as it would probably suck. The guys at id software managed to do high end 3D gaming on Linux in the past, but they also complained loudly and frequently about the state of things.
I wasn't suggesting I blame the Unity folks for not supporting Linux. Just saying that developers like Linux. Unity likes developers. Maybe they could hookup sometime. But, it might be more trouble than it's worth for them, and I'd understand that, too (it's why we don't have an installer for Mac OS X for Virtualmin, despite the fact that more than a handful of our users like having a development environment on their Mac at home that is similar to their Linux server). If it existed for Linux, I'd almost certainly give it a try; I've considered it in the past, but the whole rebooting thing just makes it less likely that I'd ever get around to hacking on any games.
Yeah, the audio thing is weird. ALSA actually has drivers for the vast majority of hardware out there, but somehow this doesn't translate into an acceptable userspace experience.
I've never used Hulu (yay, geolocation blacklisting!), but I was under the impression that it used Flash; this is probably why it's so slow. Mplayer, Xine, VLC, etc. can generally handle HD video playback on a reasonably recent CPU.
Game developers don't like to ship code for Linux. In any case the state of the drivers is such a mess that supporting a game on Linux would likely be a nightmare.
Linux support would be great for some kinds of hardware / software bundle, where you control the hardware, though.
Likely requested by the fabled "vocal minority".