Ironically enough, what you are say is happening anyway, especially with those who hold multiple acceptances to other DO schools and drop them. A lot of schools go through half of their waitlist (which could be around 150-200 people). They already have a system in place to deal with this.
The whole deposit thing is if you choose that school. If you go to multiple interviews and put multiple deposits you won't be able to go to nearly as many interviews. Your interview costs for an MD interview plus the deposit would still be 2-3 times a low versus taking a DO interview and acceptance.
I'll put it another way. Let's say you had a DO acceptance in hand. You have seven interviews lined up at 3 MD schools and 4 DO school (which you would rank above the current DO acceptance). However, you a strapped for cash and realize you could go to 3 MD schools and 1 DO school OR 3 DO school interviews (when you factor in the deposits). Which would you go to?
Most people who want to get into the best schools possible would choose the former option.