Yeah, I have got to agree with windowpain here- when push comes to shove an external burner actually does make a lot of sense particularly since the slot loading drives that Apple uses are, to be kind, not the greatest burners ever made. I had blinders on when trying to answer the specific question asked, without seeing the bigger picture.
The advantage to two drives really can't be understated (hence the option Apple provided to run two internally on the Mac Pro- the second drive was the first addition I did to mine after adding an extra 8Gb of RAM!).
Edit: After looking at the drive link windowpain included, I see by the model number that it is a Matshita drive- which are widely noted as being problematic burners. I have had three client Macbook's Matshita drives replaced under warranty because of burning issues- but of course that does not mean they are all bad!
Also looking at that info it reminded me of another advantage of using an external- most externals will be using the higher 16x-22x burning rates of almost all full height tray loading external drives available now. For comparison the internal slot loaders (like this one) will be capable of a max. burn speed of 8x on single layer DVDs. it is also does not list double layer capability, which virtually all external drives will be able to burn.