Internal burners are fast, reliable and inexpensive. External burners are slow, expensive and troublesome.
External burners are for just two groups of people: those with laptops and mechanically challenged folks who are afraid to take the cover off of their computer.
Externals make no sense for people who want to use it on two computers because for the price of an external you can have faster internals in each computer. 12X internals are going in the $50 range and you can get a 24X Lite-On delivered for less than $80. Off brand 4X externals are going for $100 after rebates. To go faster than 4X in an external you have to pay top buck for the burner and then install some sort of interface card like SCSI, Fireware or USB2.