It mixes concerns.
On the one hand you have structural matching using patterns and on the other hand you have logical matching using conditions (guards). Both are useful but serve subtly different purposes.
Mixing them is not necessarily a bad thing but from where I’m standing it’s good to know that a pattern is a pattern and a guard is a guard. I can look at a function header and know that there is no logical component to the match (such as an or) as long as there isn’t a guard clause.
