getting charged exactly for what you use is great
In the pay as you go system, you are at the mercy of the driver, and they are actually incentivized to take a longer route.
I've definitely had drivers take longer routes under the pay as you go system. I would complain to Uber and they would refund the money, but it was still a hassle to complain.
Upfront pricing is still calculated based on usage, but it sets a fair amount without giving up control and the potential for overages.