this depends on what you want-
a single mtron works very well with most normal uses-
mtron doesnt have any performance degradation ever- works the same whether disc is full or empty. no latency. limited capacity - you arent going to get your entire digital juice library on one.
but me personally- im not going to do my own shows without one - because they work so much better than any scsi or sata discs. Completely constant performance -
XServe raids are HUGE and a pain to administer.