I had the same idea but backed off after seeing some disappointing installs, but it really depends on what you are trying to do. Will it grind metal & make the surface shiner? Yes. But hitting target diameters in the tenths range for things like bearing fit ups is more challenging. The spindle is only as accurate as the bearings its running in & these motor assemblies vary all over the map. So first thing is have a peek what you have. They vary from crappy roller bearing sitting in a rubber 'centering' ring (bad), to bearing set metal end bell (bit better), to double bearings (bit better) to AC (typically only on the brushless style where they specify inclusion & even those can have issues). Under any typical grinding load, things warm up. But I have seen nice operations like grinding tapered pins or needle valve seats where a smaller motor setup is much more convenient than a big TPG.
I'm personally thinking of a good brushless RC motor because the quality exceeds these by a long shot & even smaller form factor. The ER collet adapter can similarly be installed on shaft. Drive them with an ESC & servo tester speed control. Just have to select something that matches power supply & that's where I hum & haw.