You never having seen the problem doesn't mean it never happens; I have dealt with a serious performance problem in the past that was due to excessive page fragmentation due to a GUID PK.
To your original point, these are heuristics; there isn't always time to dig into every little architectural decision, so having a set of rules of thumb on hand helps to preempt problems at minimal cognitive cost. "Avoid using a GUID as a primary key if you can" is one of mine.
To your original point, these are heuristics; there isn't always time to dig into every little architectural decision, so having a set of rules of thumb on hand helps to preempt problems at minimal cognitive cost. "Avoid using a GUID as a primary key if you can" is one of mine.