I've given this a lot of thought over the years -- in part, because I also was in a position to do something like this if I chose to.
At this point in my life, I don't really see it as either admirable (someone even used the term "noble") or sad. It's simply a choice that can have significant consequences. Whether it's a good choice depends on many factors, including (very importantly, I think) opportunity costs. For some people, choosing a "running bum lifestyle" may not be giving up very much, because their prospects in other areas are pretty limited. (I think that the dominance of East Africans in running is largely a matter of extremely low opportunity costs associated with being a full-time runner). Others who choose the "running bum lifestyle," however, may be taking themselves off a professional track leading toward great accomplishment, perhaps an obscene amount of money, and often the development of healthy social and familial relationships.
Of course, choosing another path also has opportunity costs. For me, there was always a question about how good I could be as a runner if I didn't have a very time-consuming, stressful job. I was constantly tired, and had a life that consisted almost entirely of my professional job and my athletic training. In many races that I didn't win, I was the first finisher among all of those participants (sometimes thousands) who had significant jobs outside of running. How much was I giving up as a runner by not choosing to commit to it full-time? I have no idea. In any event, the answer doesn't much matter to me anymore, because I don't believe that running a bit faster would have significantly enhanced my life in the long term.
One thing that does tend to bother me is the claim that someone (say, a 2:15 marathoner) is a "professional runner" when a more accurate description would seem to be "willfully unemployed or underemployed runner" (or, to use the vernacular, "running bum"). And all of these personal web pages detailing the training, racing, and general musings of these athletes seem over the top, at least to me.