They're not necessary, but it sure is nice to simply archive your current gemset and have at it. If the update doesn't work out, you don't have to download and recompile your old gems again. It's a convenience, nothing more, nothing less.
Not sure why you'd have to download and recompile your old gems again using plain old Bundler.
The strategy I've been using (and I think is becoming more popular) is just to install all of your gems in one big pile, and use `bundle exec` to run your app in the limited environment provided by the Gemfile.lock. So if the update doesn't work out, you just twiddle a line in your Gemfile and `bundle exec` will magically load the older version (after `bundle install`, of course, which in this case will do zero network I/O).
make sure you have Gemfile and Gemfile.lock checked into git so you can rollback if necessary. Then type "bundle update rails".
rvm gemsets are not necessary for bundler controlled apps.