An old Mac Pro will be a serviceable network attached storage device. With OS X Server it makes a pretty nice Time Machine target. The only real downside is the power draw is absurd compared to a more traditional NAS device like a Drobo or Synology. I put my retired 3,1 to service this way. If you can find some other things to do with it, it's not a totally dumb idea.
I'm also doing bonded GigE (utilizing both ports on the old Mac Pro and my new Mac Pro) which is novel but in practice not really that useful since you still really only get single GigE speeds for any one connection. It's fun for tinkering, though.
So, compared to a NAS it's not power efficient at all, it requires more administrative hassle and overhead, it's noisier, and takes a lot more space. It's a bit more flexible and can also be put to other uses if you can think of any.
Compared to a direct-attached device like the Pegasus, it's brutally slow -- but as you say that's not really a concern for some uses like Time Machine and iTunes Library storage. Plus all the downsides already mentioned. Upside over DAS is that I can put it in another room since Ethernet runs can be nice and long.