I just can't imagine that random people who play what is essentially a glorified computer game would be reliable enough to show up at 4AM and get me to the airport safely.
Or provide a reliable service in any other emergency or inconvenient situation - you don't use a taxi to go to the grocery store or to commute to work, you use it in emergencies.
I can see how an app to call a taxi and track rides is a good thing, but i can't see how having random people provide the actual rides is a good idea.
You don't think calling cabs to your location, paying with your stored card, knowing the fixed cost beforehand is a valid usecase?