JNorman, I asked myself this same question back a couple of years before I started experimenting with gear combo's of all sizes and shapes because honestly I initially felt the tapers were fairly anal and that it didn't make much sense to carry lots of extra gear when a good quality all-in-one could do the trick. Now after going through a personal adventure of trying everything out on my own, I've changed my thinking.
So for my own purposes, the answer is simply that none of the all-in-one units sound as good as the sound provided from the better sounding preamps. For me, it really all comes down to that. That said, I've turned fairly anal about my sound. So in that respect I was right...that tapers with lots of components are fairly anal about their sound. I'm not sure the typical audience member would discern between a recording made with an all-in-one versus one made with a great sounding external preamp. However, after testing and listening to alot of gear myself, I've trained my ear to know what I like and what I don't. The bottom line is that my recordings with my preferred external preamp sounds better than me to any all-in-one I've tried.