Craig, some good points, but I disagree a little..
First, using SMF's/MP3's doesn't mean you need to rely on them[believe me , we can also play anyway you want],,As I mentioned before..there are venues that we need to use this media..

Second, Arranger play is far from 100% live play. We only control snippets of data, the same data as SMF's...

As for your illustration of playing over a SMF, with vocals...Your point is not always true..
We can use many factors to make the SMF our own, and still have that "Like the record" sound that these venues I mentioned require..

Markers are one of the tools that allow us to use a SMF in a very personal way, Covers[Roland} can make a sequence sound much different without destroying the integrity of the SMF..
Using key changes[modulation] is another tool.

We have just as many tools in our arsenal for SMF's as we do arranger play..

This post is to bring attention to ways to work that maybe others[if not yourself] are not aware..