OK have thought about this and talked with Andrew.
I somewhat can appreciate philosphically what Bobby says, however I think this should proceed. The reason is that the pull of "use any docker container" is far stronger than the push back of "well you need your scm on your agent".
Some docker plugins have solved this by using a "side car" container with the tools, and maybe someday there will be support for those (as opposed to docker pipeline, which it uses now). So I think this should def. go ahead.
So - +10000 for me. The risk of not doing this is people will use bad, unmaintained images just to get the scm they need (which is what has happened in the past). They already have to have the jenkins agent running (and jvm). Bind mounting is the price of entry with docker pipeline right now, so this is how it needs to work.