When my daughter wanted to bathe alone (about 3 yrs) I taught her how to shower. I felt is was safer and that she was more likely to get clean, especially her hair. She did shower alone from then on, and never had a problem. I was always within earshot while she was in the shower and would call to her every 5 minutes or so. She didn't take baths any more after she started showering.
This is what we do too. DD wants to play by herself in the tub, so I tell her she can fill containers, but she cant fill the tub.