Well a tabletop is at LEAST 3/4" thick and most likely made either of plywood with veneer or particle board with veneer, where the balsa sheeting on the planes are usually 3/32 to an 1/8" thick, and while cracking is rare, it does happen over time. Even solid wood tabletops are known to crack and split over time. It is just the nature of the beast. The glass or cloth helps prevent splitting and cracking, the wood underneath cant move once the glass or cloth has been bonded to it properly. It isnt there to add strength, only as a protective cover over the wood. If you ding a bare wood surface, you could dent or crack it, but if it has a layer of glass and resin, or cloth and dope, it is far less likely to ding or crack. thats the protection it adds.