Ian,
Have seen the combo tees and they can work. Problem is, do they work consistently well? If you want the holes to be a proportional length, say forward tees at 60% of back tees to approximate an equal course for short hitters, then every hole should be about 60%. In the combo tees I have seen, they pick 6-9 shorter tees, presumably on holes with forced carries or long dogleg points, and move those up, but the others remain only 15 yards or whatever shorter than the next tee.
Of course, when working with an existing course, few think it is worth rebuilding all tees to get some perfect yardage. Usually, finding a back tee for length, and adding a forward tee (which often makes the old forward tee the new 170-190 yard hitter (senior men, mostly) tee, works just fine, or at least good enough in most cases.
Lastly, most courses I see could use 6-8 tees to match all the players, but whether built, or combo tees, that gets complicated, and they prefer to stick with 4-5 tees at most for simplicity.