The 11th hole at Moray Old is the one you are talking about. Very good hole because of the ditch. And excellently bunkered course, although I can't recall the bunker in question.
There are two reasons to place a bunker or bunkers next to a ditch:
1. Aesthetic scale and balance
2. As a saving hazard
Despite the fact that positioning of hazards is key to any great golf course, No.1 above is a consideration that all architects place a high emphasis on, some to the exclusion of all else.
Generally speaking, I agree that the ditch / bunker double hazard is to be avoided if possible though.
I don't see Carnoustie 18th as an example, primarily because the bunkers are quite clearly connected to the green complex rather than the burn.