While there may be external devices for pretty much anything an end-user consumer will ever need nowadays (this totally excludes the pro market), people such as myself will buy towers for one reason: self-containment. I like the fact that I can fit everything I need in *one box*. This makes moving the box, and positioning everything around it, much much easier. Could you imagine having 500 GB of external storage shared between two FW drives? That's at least two other satellite devices, excluding printer, mouse, monitor, speakers, cable modem, whatever.
With a tower, it's all there in one package. I do wish Apple would bring back the inverted-plug on the back of towers (as my 8500 had) so I could plug the monitor directly into the tower; this would allow me to get away with only two plugs on my desktop (tower/monitor and printer), instead of having to snake a power bar up there as it is for three plugs. (And that's only the bare essentials; excluding add-on's like a USB hub, ethernet hub, speakers, second printer, etc!). I don't need all of that desktop clutter.
The tower is here to stay.
Apple Towers: Heading for the Long Goodbye