I highly recommend it. I do this and so do some open source projects (e.g. jMock, Hamcrest, GWT).
It saves fussing about with separate source jars. It also means that if at some point in the future the source project is lost (as sometimes happens in large organizations) the maintenance programmers down the line will have a fighting chance of recreating it.
If you're happy that those people with access to the jar can see your source code and are prepared to pay the penalty of an increase in jar size (and a probably negligible increase in the time to create, transfer and deploy the jar) then that's all you need to worry about. There aren't any other issues as far as I am aware.
Just because the Maven standard is to keep source files separate doesn't mean you have to do it that way.