For ad hoc Rails tasks we have a few implementation alternatives, chief among which would seem to be:
script/runner some_useful_thing
Passing parameters to a rake task is a pain in the butt, to say the least. You either need to resort to environment variables or a very hackish parameter system that is not intuitive and has lots of caveats.
If your task needs to handle command line arguments gracefully then writing a script is the way to go.
Luke Francl mentions script/runner booting up Rails. That's true. But if you don't want to boot up rails then just run the script as is without script/runner. So the only real difference between scripts and rake tasks are their aesthetics. Choose whatever feels right to you.
I use rake tasks for little tasks (one or two lines). Anything more complicated goes into the script/ directory. I'll break this rule if I think other developers will expect the code to live in one place over another.