A few months back we built a 4-Tesla box for doing GPU cracking (I think for less than 15 grand total). Initially for doing WPA, but now for more general purpose cracking.
It's ludicrous how much quicker we get results. I'm hoping to start compiling some statistics, as it's been in near-constant use since January.
We don't normally crack hashes for proof-of-concept, it's not usually that hard to convince a client about the perils of insecure password storage (although I guess ocassionally you might need to give them a concrete example).
Generally, we crack passwords to be able to try those credentials on more secured systems (thanks to the ubiquity of password reuse).
As to why so many, When doing network tests, there's some percentage of time when you manage to obtain a hash (it might be a small percent), but when you multiply that by the number of tests going on in a given week, it seems to have worked out that folks are lining up to use it.
It's ludicrous how much quicker we get results. I'm hoping to start compiling some statistics, as it's been in near-constant use since January.