I actually use my local rdoc all the time.
I followed this guide http://alkesh.vaghmaria.com/2009/07/offline-gem-server/ to have a local rdoc server running through passenger (so I don't have to run gem server manually).
This is great for working offline, and it also means that the docs are always usign sdoc, which is a great template which allows for good searching. >90% of projects have crap rdoc templates.
I do agree that building ri is a waste of time, though.