My two girls, ages 6 and 9, still bathe together. Why? Because they still like to. I see nothing wrong with it whatsoever. I'm glad they still like bathing together and having fun. They grow up so fast.
We just moved into a new house last year and our master bathroom has a BIG whirlpool bath so they LOVE having baths in our tub!
As long as the kids don't have a problem with it, why should anyone else?
It saves time, water and they have fun too! It cracks me up what games they invent.