gemsets are especially important for beginners that will be trying out lots of different codebases with different gem dependencies.
With rvm you can create a gemset just for that project youre testing out. Imagine trying to work with different projects that have different versions of ruby, different versions of rubygems and different versions of rails, some gems from github, some gems from source, some gems from rubygems.
A beginner would not be able to sort through the dependency mess this creates. Gemsets help you avoid this nightmare.