I use my two TC's exclusively for backup only. One is dedicated to my iMac, and the other is shared between our two MBAs and (now decommissioned) two MBPs. They work great for that purpose. I am not comfortable sharing my backup drives with local NAS drives. I dual back up my computers... once to the TC via TM, and my primary backup is to the cloud using Crashplan+.
I have an 8TB HP MediaSmart server (WHS) that I use for shared storage in the house... although I wouldn't buy a new WHS now that MS has killed their drive extender technology. I would probably use DROBO now... although I have mixed feeling about those as well.