oh just thirty years of experience.
A pencil eraser wont stand up very well to repeated hits. You can easily tear an eraser with your fingernail...what makes you think it will stand up to repeated hammer hits?
try it before you say it won't work.
the erasers I use are the bar style cut with a plug cutter and glued in with super glue. in the old days I used the wife's nail polish as glue. sometimes I used the erasers that were designed to erase ball point pen ink but they are hard to find now a days.
these home made snap caps usually last for about 85 to 100 strikes. I use em when doing a trigger job, to verify mechanics, and sometimes to get used to the pull. I am a cheap bastard and these caps are lots cheaper than store bought one.