Thick impasto in oil paintings requires more curing time before varnishing, for the best results, but yes, they should be varnished, otherwise they'll deteriorate much more rapidly. Too much impasto is problematic in a number of ways. It creates surface convolutions that make paintings very hard to clean, for one thing. Also, in thick passages a larger amount of the binding oil will migrate to the surface and dry there, and then eventually will darken and change the way the colors read. Impasto as used by the Old Masters was reserved for certain places and used for certain purposes, rather than applied randomly. It tends to call attention to itself, and thus can serve as an accent to direct the viewer's gaze where the artist wants it. However, this device is squandered and rendered less effective if it is used over too large a percentage of the painting's surface. It is most effective when juxtaposed with thinner, smoother areas. In the works of the Old Masters, the paint is thickest in the more important highlights, and thinner by degrees everywhere else, depending on how much light is being represented in a given area.