You mean if there's no safety reason not to obey? I'm thinking a positive attitude towards safety and regulatory compliance, because I've never heard of a controller giving such an instruction given without a good safety reason like keeping planes from banging into each other.
Usually not, but I've heard it happen.
Because the pilot says "Unable, traffic/clouds below me"?
Look folks, it's a team effort here. You want to play "I've got a secret" with the controller by just saying "unable" without explanation, you need to expect the controller to get cranky with you since you're now making him/her work harder, and if multiple exchanges become necessary, you're hammering the bandwidth available. Likewise, pilots must realize that controllers don't do things just for fun -- their primary concern is safety, and they aren't going to give VFR aircraft instructions in Class E airspace unless they have a good safety reason for doing it. The Chief Counsel seems to understand that concept even if some pilots here don't.
Personally, I trust controllers (within Ronald Reagan's dictum to "trust but verify") to keep me safe, and they trust me to do what they tell me so they can keep everyone safe. What is so bloody hard about working together to accomplish that?