That is a good point.... Though, i'd guess most use glycol jackets as their external temp controller to individually control multiple fermenters and not just controlling the air temp around the fermenters? I could be wrong on that though. I kind of assumed the glycol jackets would have more cooling power than just the outside air, which is what got me wondering about the internal coil to give it a bit of a boost during the initial aggressive phase, but I could be wrong on that.
I suppose the tricky part might be if you have 2 or 3 going at once that might need slightly different requirements between them. When we have done that, we would just measure the air temp, which, yea, isn't ideal, but figured with smaller batches, the air temp, with a small fan to move the air around the box would be close. I figured with larger batches, it would be less "close enough".. heh..