I think it has a lot to do with people thinking more=better and spending more $$=better. "If I have a $1000 outboard preamp to record PA systems, it will DEFINITELY sound better."
I semi-kid, of course. If I recorded symphonies or nature material or stuff for film, maybe I would consider a really stellar outboard pre (though really, I am at a loss as to why people think the SD pres aren't "good enough"). But I don't. I record rock music. I also live in New York City, so I carry my gear on public transportation to every show. For me, having an all-in-one box makes a hell of a lot of sense in terms of size and weight. I have the R-44 with the Oade "concert" mod and I think my recordings are quite good. And frankly, based on my buddy running similar gear, I'm not sure I really needed that mod, either.
In terms of sound quality only, I really don't see the point of running mics>high-end pre>analog inputs on M10.... even if at the mythical "unity gain" it seems to me you are introducing the analog components of a rather cheap (albeit, well-made) piece of gear into your chain. Does it actually sound worse? Probably not. But if you are the person who thinks a $1000+ pre is necessary, it would surprise me if you couldn't "hear" the M10 in that gear chain.
It seems to me that if you want to go the typical route, something with digital inputs, like the Sony D50, makes more sense for this type setup.
But this is all just opinion, of course. Lots of good recordings are made all kinds of ways.