Fact is you can get a DVD burner (ok, not the fastest, but still good enough) in a Mac mini.
Those ext. HDs are there not only for extra storage, but also for having a faster HD.
One of the most "crippling" speed factors of the mini is the very slow internal drive. It is only 4200 rpm, while 7200 rpm is considered normal for a desktop. If the mini is your only Mac, then getting an external FW HD is pretty much essential for the speed. Especially for swapping memory (which OS X does quite a lot if you don't have tons of memory)... higher rpms mean better performance.