I just finished building up an old G. Fisher, fully rigid, frame not to long ago. Granted I'm still getting the bugs out of the shifting system (mostly because I'm using old parts mixed with new and they don't seem to want to play together nicely) but I'm riding it and it's great. I agree about the maintenance issue. If I'm out on a recreational ride and my bike breaks down, it's inconveniant but not hurting anything. If I break down on the way to work I could lose pay, maybe even my job if it happens enough, and piece of mind because I'm worried about the above. Usually you don't care about the "potential" for break down until you do and then from that point on you always worry about it, at least a little bit. More so if its a major component of your bike and you have to gerry rig it to make it rideable.
If you do upgrade I would strongly suggest a touring/street (flatbar tourer) bike. Almost all of these I've seen come with fender AND rack mounts as well as road bike sized tires which can make the commute faster. You also tend to get higher gearing and less "flashy" parts which add to the speed and durability of the bike in the long term. Example...most touring bikes come with square taper bottom brackets which have proven themselves for durability and longevity. This also helps your overall costs because you won't have to replace parts very much like you probably would having a light XC full suspension bike.
I've never found my fully rigid bikes uncomfortable and I commute in Michigan and we have TERRIBLE, TERRIBLE roads. Lots of pot holes, cracks, "almost" pot holes, and bumps everywhere. I also usually choose frames that aren't a dedicated racer as these are optimized for, well...racing, which usually means stiff, stiff, stiff which equates to "feel every bump, feel every bump, feel every bump" when commuting. (I usually also usually choose steel which automatically makes it more comfortable than AL or Carbon....hmm, hm, hmm, ha, hmmmm.....(running away very fast now...hehe))
