I think that an external clock is only necessary in one of the following two situations:
a. The internal clock is crappy compared to the external or
b. You need several digital devices connected together.
I read a fine article by Dan Lavry the other day, about the misconception that some users have about external clocks. The question was, whether or not external devices (H-8000FW included of course), will sound better when clocked from a dedicated external clock, like the Big Ben for instance. I enjoyed the article a lot, since it provided great insight as to what the situation really is, regardless of the marketing strategies and the fuzzy words of some companies who are just interested in your wallet.