Maybe you haven't gone through enough of them to know then!
I've run into some real beauts for cheapo mice over the years that simply fall apart upon use!
Intermittent problems can also come up with ports as well as mice. You can easily stretch standard PS/2 and usb ports alike when moving a case around and seeing the cord pulled sideways while plugged in seeing some damage that way as well.
If the contacts in the mouse itself become worn or polluted with debris that will also cause problems at times. Even optical mice have to be cleaned just like you would with a mechanical mouse to see those work well. Trying out another of the same type of mouse will show right away if the problem is the mouse itself or something else needing to be looked at.