"rubella vaccine is an advantage to your child as they avoid an unpleasant illness"
Rubella is very very mild. A cold is more unpleasant. Chicken pox is much more unpleasant and lasts far longer than rubella.
"They're not pretending that the rubella bit of the MMR will directly benefit the child..."
So why exactly should my 1 year old baby bear the responsibility for an unvaccinated mum? Should she not be responsible for the safety of her unborn child?
And why should my 1 year old baby boy be vaccinated for rubella at all?
"... (although if your child is a girl and gets the vaccine, I'd argue that it will directly benefit her in the future if she gets pregnant)."
You seem to be under the impression that a vaccine at age 1 will protect a girl when she is pregnant at age 20. It won't. These girls will have to have boosters through their lives, never completely sure if they are completely immune (vaccines don't confer 100% immunity).
The better choice for DD is to have rubella as a child and be immune all her life.