Heres my experience...
I kept my TC HDD mounted all the time so that I could have free access to the files without a lot of hassle. It also contained my Itunes music folder, which I used daily.
I just got the replacement for my TC because my hdd failed. Ive had it since March of this year...it made it 6 months...Looking back, I wander if streaming itunes, and leaving it mounted all the time and writing files back and forth caused it to die prematurely...I dont know that it should though...