I would pay extra because I don't want the cognitive load of haggling on the cost. When I want/need A ride, it's because I'm either overloaded, away from home, or drunk. These are three times that I didn't want to bother with any more nonsense.
Having used it in Mexico, they usually have a suggested price that you can just pitch with, and usually people would accept that or add a tiny amount on it.
In a system where you are expected to haggle taking the opening price puts a pretty heavy tax on everything. It's more work to get to a vaguely similar equilibrium distributed slightly less evenly when you look at big markets for similar things.
