Having a projector of your own that works well with your laptop can save time and hassle when you want to set up a presentation at a remote location. Small projectors can be fun for watching movies back at the hotel, too.
If you value portability most, consider the Optima PK 102, which is about as big as a deck of cards. With 4GB of storage and built-in presentation software, it can function as a standalone projector for even fairly large presentations. It's bright enough for dim rooms and medium distances, but when we used it to project at distances of more than 6 feet from the screen or wall, it produced weak images.