The top mount wears due to it being a steel/rubber bonded unit and should be replaced when carrying out suspension work in the area. What does seize and become stiff in operation is the bearing attached to the top mount. The unit will not fall off but depending on it's state of wear, may break apart.
It is not a very complex job - replace all nuts and bolts whilst you're at it and choose a suspension set-up that suits your driving style - lots of options on this forum. I have two cars in the workshop at the moment requiring this job and both owners have selected very different set-ups.
blue skies
tony