Hey Bigfoot,
Foil on the bottom will keep your life simple. Especially with larger cuts, like butts and briskets, there's a LOT of drippings! Due to the heat from the element, those drippings will bake very hard on the bottom without foil. Yes, you get a little grease under the foil (I think mostly from the moisture that flows down the sides), but it easily wipes clean with a paper towel. If you bake those drippings onto the bottom, cleaning is going to require a lot of scraping! If you don't clean the burnt drippings, you run the risk of fire, and nasty burnt smoke on your next cook.