Why commercial support needs to solve your customers' problems
If you're providing commercial support, I think that it is very important that you actually solve your customers' problems or, if you cannot, at least realize this very fast and tell the customers.
(It is not so much because who get nothing out of their support contracts will stop paying for them; indeed quite possibly not.)
If you do not actually solve their problems, your customers have become your unpaid debuggers; in fact, they're paying you for the privilege. This is because going through the work of bug and problem reports only benefits your customers if you then solve the problem. If you do not, all of the effort the customer has put in to the process only benefits you (in the long run it helps you improve your product).
(This is the corollary of who benefits from bug reports.)
This matters a lot because humans fiercely resent being taken advantage of (a behavior that seems relatively hardwired). Your customers may dislike spending money and receiving little value, but they are likely to hate you for being taken advantage of, whether or not you intended it and whether or not they realize it.
(This is why it is important to fail fast for problems that you can't fix, before you have the customer put in a lot of work doing diagnostics and the like.)