...and I'm getting really tired of seeing people try to look good by saying "look at me, I'm anti-anti-establishment!". Several things spring to mind concerning your argument:
1. A bigger market share does not ensure more viruses. There's no point in writing viruses for exploits that don't exist. The viruses exist because the holes are there, not just because the OS exists and it might be fun to throw rocks at it. And if it were primarily a numbers game, then shouldn't there be a lot of viruses out there for the areas that Microsoft doesn't dominate, such as web servers? Many (if not most) of those are hosted on non-microsoft platforms. Worms exist for the server software (php, apache, etc.) but not many are targeted at the OS itself, except for MS.
2. You're right concerning security through obscurity. However, what's more obscure than window's source code? Shouldn't it be easier to write viruses when you can go through the code for problems?
3. Security and usability are diametrically opposed? So, should we just give up? What about secure, documented, and validated software? These two things are only opposites when people spit out some half-assed software, with no foresight or emphasis on security, which is what MS did for a long time. Now they are making some steps in the right direction, but sp2 broke several apps on my machine. People say it wasn't Microsoft's fault, but what does that say about the security of XP when it shipped in the first place? And many of my problems affected hardware that was "XP Certified".
Now, I'm not saying that linux or BSD or whatever has all the answers. Maybe MS can buy their way into some good technology, and provide some good security with their plain old basic Home edition, or whatever they are going to call their entry level OS when they finally release it.
Oh yeah, and to answer your question, yes, I do think that a computer should be sold to me that is secure. What's more, your "Joe Sixpack" thinks it should be as well, he just doesn't have the time or energy to mess with it.