I think it depends on how much exploring you want to do. The resort location is a bit isolated, but taking a taxi to the shopping areas wouldn't likely cost too much, when you wanted to go. The resort may even have a shuttle.
I had no issues driving anywhere in Mexico - if you're a smart, defensive driver, and understand that the "rules of the road" are kind of slippery, then you'll likely be fine.
The larger issue for me is the extra costs of a rental car in Mexico. Even if you start with a great base rate, the added third party insurance tends to drive up the cost a fair bit. You'll have to weigh the cost per day for having a rental, and whether you really need that car sitting there unused most of the time.
All things considered, for my money, I'd skip it unless you really think you'll use it.
Dave