I think it's as valid as any other kind of photography. I respect and admire the amount of effort it takes to be successful in that genre — not only does one have to be "on time" to the second, one has to be equally adept in research and knowing things before others find out.

I don't think there can be many who find it a lucrative way of profit — there are only so many sites and magazines who need such photos, and many of these photos lose value over time (exception being photos which capture someone doing something that genuinely won't be forgotten for many years to come).