Hi Marc,
I almost always create a custom entity for Projects. If you repurpose Cases (or some other entity) not only do you have to get rid of things on the form that you don't need, but you have to be sure the client will NEVER need the case functionality, as it stands. The primary thing to consider (I think) is the rollup of activities. Activities rollup to the Contact/Account in a unique way with out-of-the-box entities that cannot be recreated with custom entities.