And how many times is this the case? Plugin vendors which sell obviously crashing plugins lose business anyway and get a bad reputation. Also if someone buys before trying and then continues doing that with those plugins, then he/she* is just stupid. In that case there is no such test tool needed.
But this is not what happens most of the times. 90%+ of the times these crashes are caused when doing a certain combination of things on the plugin GUI, or just sporadically (it seems). There are also cases, where certain plugins have issues only in combination with other plugins loaded in the project. This application is fine for general testing, but not really for real life use.
Also it is a dev tool, usually not meant for the normal user and also not easy accessible.
The big issues are not caused by plugins which clearly crash, these are easy to sort out, instead its those times where Cubase closes out of nowhere (like it is the case here), often without a crashdump.