I develop on a Mac, and I'm pretty comfortable setting up an Ubuntu VM with Apache installed, so "you don't need to install Apache" is not really a reason. I run Ruby from the VM anyway, so there doesn't seem to be much difference in system resources needed.
In my head, the advantage of using Apache is that it's going to perform more consistently with my production server.
So what reasons are there to use WEBrick? Does it make development easier somehow?