It is not self-powered all the time - indeed, some 10,000rpm small footprint laptop drives that go in these things consume 1.5A's - the rating for USB (1 or 2) and Firewire sockets are 500ma each. So, to power a new fast drive in these things you need at least 2 pwr sources, which can be 2 usb ports - even then, that's only 1A, still 500ma short, and you risk damaging whatever PC you plug these devices into. Plus, there's the worry of heat build up within the case - some rely on the case being a heatsink, but that is not good enough, especially if there is no proper insulation (risk of shorting out depending on where the caddy is placed).
We have them at work, or at least some editors do, and they plug them into non-linear editing stations. Media can be consolidated and played back from drives in these enclosures on USB2.x - but, we warned the editors that if they break the PC ports, then they pay for a new crate (some ports are on the MB direct). We know who has these devices so there's no escape