The reason you don't use tail sitters is because they have much less payload capacity. In that configuration the engine has to be able to output thrust greater than the entire weight of the aircraft. Something like the 737 has a thrust to weight ratio of only 0.25 (you need greater than 1 to take off vertically).

A more direct comparison: A fully loaded standard F-35 has a thrust to weight ratio of 0.87 (that is with the afterburners on). In comparison the short take of and landing version of the F-35 loses 43% payload of its and 25% of it's fuel capacity (and thus range) [1].

1 - http://en.wikipedia.org/wiki/Lockheed_Martin_F-35_Lightning_...