I guess the difference is that exho's are obviously nowhere near as demanding on the body and considering how Andy played during Boodles, probably a great time to relax and just enjoy their tennis without worrying about ranking, money or reputation.
If Andy's already in Miami, then playing a maximum of two matches in an exho is not going to be demanding at all. No extra flights, or dealing with jet-lag, or getting used to a strange hotel bed.
You could argue it might interfere with the off-season training schedule, but I can't see it being that hard to rearrange. He wouldn't want to be doing any massive endurance testing runs the day before, but he could have a complete week off, then a week of gentle training, and a few days slightly more serious, then use the exho for a bit of hitting practice!
Andy also has the advantage of getting to spend the tour finals week in his own home. Poor Del Potro has a much longer commute! I think he's doing an exho in Argentina, in November, which seems very sensible. But I also read Federer is meant to be doing exhos in South America (can't recall where or when), which is bonkers if it's true.