I need to show that several systems are interacting with each other but without going into too much detail. I've decided to use a use case diagram so my question is: how wrong would it be to have an association relation between actors (interfaces) as well?
I cannot imagine the purpose of drawing direct association between actors. That would mean that an association exists between actors but the association is outside the modeled system. So it is just a note that has no impact on functionality of the modeled system. I would say that the actors are usually linked by using one common use case - that would be the association inside the modeled system with some impact on the functionality.
I personally prefer to associate use cases to processes rather than actors.