Really? You're drawing that conclusion for everyone?
No one "needs" to go scuba diving, in fact no one "needs" to go to Cozumel at all. People go on vacation and do things on vacation that they "want" to do. And some want a rental car to have the freedom and flexibility to go anywhere they want at any time they want, without waiting and without having to always have proper small peso notes.
I'm assuming you were in a group of 4 so that was $80 for the taxi. That cost will be the same for just a couple so twice as expensive. Also, I've never seen such a low cost. The usual quote in Coz4U forum is around $120 for the day. I've asked a taxi before and was quoted $140. Can one call the taxi company and book this $80/day taxi? Not everyone wants to haggle with a taxi driver.
All that said, I'll agree it's best to get a taxi if you want to go barhopping and drinking because driving drunk should not be on the list of options.