This is noething new. (Score 1)

The best reason for doing it is to get a case that holds more that the 1 or 2 externally accesible bays that Apple thinks is enough. I've done this before myself, but even cheaper. PC case for $30. Apple AGP logic board for $100 from ebay. Apple power supply from ebay $100(you can also modify a plain old PC PS) and a Sonnet 800mhz G4 new for $299. And mine has 2 Pioneer DVD-R drives, a 52x CD-R a Sony 5.2 gig MO drive, and, ready for this?...A 200mb Syquest drive!!!!
All internally mounted-that is why this makes sense.