C'mon, chas..! Give us all a break, here!
'Amateur hour', indeed

First of all, this is the "general' arranger forum, NOT the 'pro' forum, so there are LOTS of 'amateurs' here in the first place. Secondly, this is an arranger forum, so let's face it, the primary thrust of this place is for players who DO use some kind of artificial backing. It certainly isn't a 'pro' KEYBOARD player hangout (you know, the ones who play with bands, exclusively - there are plenty other forums where you can go and poke fun at players that use machines and be in like minded company!)...
Now, you know my feelings about mp3's, but they primarily apply to the worry that, after making them from SMF's or live players, whatever, if you upgrade your keyboard, the backing still stays the same, so not much change to your sound. And they lose the flexibility of being able to mute parts if a player sits in, artifacts in the sound if you transpose them or change their tempo's, etc., or just generally mucking with them while you play.
But the mp3 is, in effect, no different to an SMF backing. How much, or how little is in them is entirely up to the player (if he makes them himself). Bass and drums and that's it could be all they contain.
Now, I know you don't play arrangers out at all, but probably most here, if they gig at all, probably gig with the arranger (makes sense, given the forum title!), some use just the arranger section, some just SMF's, but most probably a combination. And mp3's, for all their problems, can be used pretty exchangeably with SMFs for backing.
I know you only play out with full bands, but in today's economical climate, most of us don't have the luxury of saying that, if the band can't find a booking for tonight, we'll just sit at home and collect Social Security

We go out and do solo work, duo work, whatever we can find. And this entails some backing, if the venue wants more than a cocktail pianist. Arranger, SMF's, mp3's, it really doesn't matter which, once you decide to go down the autobahn.
So give a little slack to a keyboard that includes all three methods of backing a player... not all of us are retired and can pick and chose our gigs and band personnel with no financial considerations!