Yeah, I think trimmers are usually a cost cutting device in a design. You put a few in your design to allow you to compensate for wide tolerances of cheaper components and less stringent assembly requirements. Instead of paying the premium for 0.1% resistors, top notch soldering jobs, and 2oz copper (for example), you spend a little on trimmers and you go with 5% resistors, regular soldering, and a 1oz board and just pay someone to tweak the trimmers in the final testing stage (which you're doing in either case). Or instead of using a controlled impedance substrate and measuring your hand wound inductors, you use FR4 and 20% inductors and just trim it up to spec (again not a fantastic example, but the point is there).
I think the difficulty of a stereo trimpot lies in manufacture.... they're not super consistent because of the mechanical wiper aspect, and with trimmers, generally your adjustment range is smaller and using a smaller range will be harder to match between the two channels. Then if you get like two 1% trimpots in one package.... how do you make them track to the same value? Even if they track completely linearly, you're likely to get one that it slightly offset from the other, so you'd just need another trimpot to compensate for the offset thus defeating the purpose.