Choosing between patching old plaster and covering it with drywall

The bedroom walls have old plaster with hairline cracks and a few uneven patches. Most of it sounds solid when tapped, but one area near a former repair is crumbly. A contractor suggested covering everything with drywall for a smooth finish. That seems quicker, though it would bury the original surface and alter trim details. If the plaster is generally sound, is patching and skimming worth the mess? I'm trying to distinguish cosmetic cracks from a reason to replace the surface.