I get about 100 blades on a hard steel platen before needing to flatten it, and I don't use a good platen for profiling. A well used hard steel platen is shiny and smooth without ruts or dents for several months of production grinding.
The grinding belts we all use are designed for different applications and have been adopted by knifemakers. Some are intended to be used on centerless grinders and a contact wheel, some should not be used wet or damp etc and can form abrasive rubbery black crud that sticks to both the platen and the belt. If you see buildup forming on the platen you're getting it on the belt too and that buildup will contain abrasive grit that was formulated to cut hard steel. Under some conditions a hard platen can show damage in a single belt, it just depends on the grinding conditions.
Just to be clear, everybody here knows I make hardened platens, and I'm not trying to say that a hard platen is a panacea for all grinding problems. But a hard platen does grind smoother (lower coefficient of friction) does hold its shape longer and conducts heat and static better than nonmetallic materials. But eventually you'll need to mill, lap or grind it flat when it gets wonky.
A good application for a steel platen plays to its strengths such as the ability to machine a radius into the end for grinding the plunge when grinding lengthwise and the ability to conduct heat when cooling with ice water. A bad application is simply slapping an expensive precision made platen onto a grinder and beating the piss out of it. For that, someone should use something hard and disposable like glass or ceramic.
