I see little difference in whether your backing comes from you having prepared it in advance. and you play full two fisted keyboard over the top of it, or you play it from triggering the arranger with your left hand and play ONE fisted over the top of it..!

IN NEITHER of those scenarios are YOU playing everything that comes out. Your contribution is the stuff you play LIVE. Looked at this way, for all but the best arranger players, that's HALF of what you could be playing if you used SMFs...

Now, sure, you can re-harmonize, rearrange, change song structure, all on the fly with an arranger. But, let's be honest with each other... How many, other than the top few arranger players, actually HAVE the knowledge or skill to use chord substitution effectively? Darn few, I'd be prepared to say.

'Ah, but I can change anything I want in the middle of a song', sure you can, but you must be careful... Half the 'cheese' I've heard coming out of arrangers has come from some using this ability poorly, and making on the fly decisions I'm sure they've regretted later! In all fairness, there's only ONE thing the arranger can do that an SMF can't... Cover for YOUR mistakes! I'm sorry guys, but one of the things I hear a LOT of players with arrangers do is screw up the chords, or structure, get lost in the middle, miscount a section, whatever, and the arranger keeps patiently chugging along, making the best of what, in a sequence, would be a total train-wreck! Maybe this is why some refuse to countenance SMF's, in truth! Screw up a sequence, and you are toast!

But, OTOH, arranger SMF players now have Mark/Jump abilities, that allow song restructuring on the fly (used to be an arranger mode exclusive), the Roland's, at least, have their Song Cover abilities, to allow a complete re-voicing of the SMF (change a rock band into a jazz band, for instance), once again, formerly an arranger only ability, and dual sequencers or pretty instant SMF loading can give you the 'medley' ability that used to be another arranger exclusive.

Used carefully, most of what USED to be arranger mode's exclusives now CAN be done, at least a certain way, in SMF mode, and some of it can be improved (FINALLY, you can have bass-lines and other Parts that LEAD the chord changes, not just follow them!). But one thing the SMF gives you is the backbone of being able to play with others... The ability to play well with BOTH HANDS...

Particularly if you are learning on an arranger, or are just getting back into music after a long break, this MIGHT be a skill worth working on. You know, just in case you get an opportunity to play with something OTHER than a machine!

Arranger mode has it's charm, and skill-set, and done well is very effective. But be careful of dismissing the humble SMF. It has powers that many do not realize it could do now, and it DOES give you the opportunity to DOUBLE the live playing that you are actually playing (and trust me, the audience WILL notice that!), and practice up skills that will pay dividends if you ever sit in with a live band...

First of which will be the discipline to learn your tunes correctly... because a live band MIGHT hang around while you miscount a section or play something wrong, but you won't like those raised eyebrows as they do!