how many of these women had 2xMMR?
As for an unvaccinated child catching rubella from a vaccinated child, I want to see the lab conformation for both, also no one said vaccine induced immunity was 100% (especially after one shot), but on a population level, you can eradicate rubella if everyone gets 2xMMR. It's not like the vaccine has been invented last week. Different schedules have been tried in a number of countries and we know that not vaccinating doesn't work, and vaccinating only girls doesn't work, but vaccinating all kids does (and then, even if there were the odd 40 year old losing her immunity, there wouldn't be a child to infect her during pregnancy).
Is it wasteful to vaccinate a child who has had rubella with MMR? No, it is frivolous to allow your child to provide breeding ground for a vaccine preventable disease. If a rubella immune child gets the MMR, the M and M are going to do their bit and the R is just going to get neutralised by the immune system. If the child's titers were lowish, the R might boost the child. Not wasteful - efficient. I am old enough to know the girl with full blown CRS, my great-uncle was "only" deaf. I had a colleague who had an abortion because her son infected her with (confirmed) rubella in pregnancy. Both situations are entirely preventable.
LeVolcan - in Germany they don't routinely test for rubella immunity before you are pregnant. My doctor did that on her own initiative (and that was not in Germany). The idea to screening for immunity once the woman is pregnant is silly (at least in an environment in which rubella does still circulate - we had two cases in DD's kindergarten in Germany in the two only unvaccinated children, two siblings, probably picked up from the Steiner community down the road), after all, you cannot get the MMR during pregnancy.
That doesn't change the fact that 2xMMR on a population level will get rid of all indigenous cases of CRS and that, in my opinion, is something to strive for.