Well, it's still kind of wasteful. At least IME I never install more than a modest fraction of all the packages at all, but if you have enough same-arch machines it could still be a win. I'd have to have copies of three releases here (32 and 64 bit x86 of Ubuntu (and let's not forget that there are machines on different releases a fair part of the time as well) and x86 Debian), nah, it would be silly... for me.
Sharing apt's cache over NFS might run into concurrency issues, but if you're the only one who admins any of the boxes, okay, that probably can work. (I remember setting up NFS for no reason but to be able to run the Slackware install without having to have a great pile of floppies...) One thing I don't believe that addresses at all is purging obsolete versions of packages as security and bug-fixes roll in.