I'm not keen on the idea of players playing along to commercial MIDI files. I know some people have fun doing this at home and that's perfectly OK, but it makes me angry to see so called 'professional' entertainers doing this for a living, just adding a few right hand notes to someone else's work. I was once asked by a fellow 'organist' on the concert circuit if I could provide her with backing files for her performances. I declined and I'm sure that everyone else she asked did the same. Her time on the circuit was, unsurprisingly, very short!
But I have absolutely no issues with someone working hard, creating their own MIDI files and then multi-tracking with them in concert or on recordings. Multi-tracking has been around for years. I do like people to admit that a piece is multi-tracked and I'm a bit disappointed when someone says that 'everything is live', when I can see and hear that it clearly isn't.
However, my point is that there is no need to do either, if you have the skill to do so. Turn off all the gizmos and play everything yourself! There are plenty of 100% live Electone and Atelier clips on line.
I repeat my earlier comment that when played live - manually - with no automation at all, the organ can do things that the keyboard cannot. It's down to the skills of the player. I'm biased, I've been playing organ for 48 years and have had the privilege to know some of the best artistes on the planet, alas many no longer with us. I'd rather sit down and play organ than any other instrument. I don't care if it has pipes, transistors or microchips.