Why use homemade toothpaste? Some do so because it's cheaper. Some, because they want to avoid the flouride that's in most commercial toothpastes. Others, because it's been shown to work extremely well (in combination with proper diet) in resolving gum disease (gingivitis, peridonteal disease).
I did find one site where a dentist reports that using baking soda is bad and recommends Crest Original Toothpaste. You decide, Dear Reader.
Here are some sites with several interesting recipes on how to make homemade toothpaste and tooth powder:
Tammy's Recipes - Toothpaste
(including cool flavors like cinnamon and peppermint)
Passionate Homemaking - Toothpaste
(using Castile soap in several versions)
The Organic Sister - Toothpaste
(she includes aloe vera in her recipe)
Idea Queen - Toothpaste and Tooth Powder
(orange rind, sage, even powdered chalk are ingredients here)
Green Eco Services - Toothpaste and Tooth Powder
(hydrogen peroxide, salt, goldenseal, and lots of other interesting stuff in these recipes)