I have 5 external bus powered 2.5" enclosures and one bus powered audio interface.
Everyone's darling M-Audio Firewire "whatever" can also be bus powered, so ..
Of course, almost any of these, with exception of LaCie Rugged FW800/USB3, could be powered from external PSU, but who wants to deal with that, right?
Besides, despite the presence of +5V DC IN input socket, inXtron 2.5" enclosures, that I have, were sold without the external PSUs. From now on I personally will be very cautious when connecting anything bus powered to the older Macs.