I totally follow you Dave. We need this. I do think the functionality we already have for holding E (and holding D) is good and fits Animation Paper as it is. But we could do this temporary change to an eraser using another key instead.
Actually, the three erasers and the three drawing tools are on keys 1,2,3,4,5,6. We could make them work as you say. So hitting, say 2, changes the tool to rubber eraser. But holding 2 changes it temporarily to rubber eraser and springs back to the tool you had before when you release the 2 key. All 1 – 6 could work perfectly this way.
What do you think about that?