RMagick is a leaky, slow and unmaintained piece of software. It is also used only for small retouching on images, the primary use-case being to make thumbnails. PIL on the other hand is more reliable and can do things that RMagick does not, like actual drawings or to put it another way, whatever you can do with GDK, you can do with PIL, all in one nice package.
PIL is not perfect either, but there's nothing like it for Ruby.
I can't argue about it not getting much love in the updates department and its instability under many configurations (though sometimes you can get lucky ;-)), but..
and can do things that RMagick does not, like actual drawings
RMagick has a SVG-related drawing system called RVG:
Isn't RMagick "like" PIL?