It depends on where you live. In New York or Chicago, you just hail a cab, usually in a minute or two. Pulling out an app and trying to get the marker in the right place and waiting 5 minutes is a step backward for the kind of usage you describe. An app is a very clumsy replacement for street hails.
For me this goes doubly so when I happen to be walking somewhere (in NYC) and see a cab driving past with its light lit - I might have normally walked the 15 blocks, but since there a cab right there, and its available right now.... its almost like an impulse-buy on the transportation.
