Chas, I think you are ignoring several more recent features that makes using automatic rhythm section stuff FAR more flexible and capable of going off the page. Obviously, for starters, there's using arranger mode, so chord substitutions, key, and structure can be totally changed on the fly.

Then there's the dynamic duo... Using Markers in SMF's so that a static song structure can be completely twisted around on the fly depending on needs, extended, shortened, extra vamp sections optionally inserted, and the Chord Sequencer, which allows for capture of a basic 'head' chord structure, and then allows hands free repetition of it while you jam two fisted to your heart's content, all the time still being able to fill and change variation and key, etc. on the fly (hands free again with a footpedal)...

Add to that, at least with Roland's, the ability to totally revoice a style from hard and rocking to soft and jazzy on the fly, with a mere button press or two. Same style, several orchestrations...

The only thing holding you back may be unfamiliarity with these tools, or unwillingness to try them. Sure, if you have an on call rhythm section at your beck and call, willing to play for peanuts (or free!) 24/7, you are good to go. But when this dream doesn't pan out, don't dismiss the machines so lightly. If you WANT to have a lot of on the fly control, the gear CAN do it. You just have to use the tools...