Having your own license (usually a
variant on one of the existing ones)
allows you full control over it. I
suspect that things like the huge
change between GPL2 and GPL3
(especially if you deal with web
services) make people leery of
licensing with an agreement outside of
one's control.
I think this is true to some extent. However, when you license under the GPL, you have the option to say, "Version 2", "Version 2 or later". If you're leary about how the license may change in the future, you are free to just say it is licensed under that specific version.
