It depends on what you shoot, and what type of output you are targeting, and what you mean by "pro".
Plenty of people make good money shooting crop cameras. In fact, Canon's "pro" camera for sports is the 1.3 crop 1D Mk III, which is also a premier camera for many other "pro" uses and has only 10 Megapixels -- but they are capable of producing images that are tops in IQ..
On the other hand, many pros in the fine arts (large gallery-quality prints), in weddings, portraits lean toward full-frame to squeeze out the maximum amount of detail and image quality that the ff cameras can excel in. Also, many publishers require high resolution files that, if you were shooting with a crop body, you may struggle to meet the requirements.
So, there is no pat answer. Don't get an expensive body just because "the pros use them", but determine your specific needs. Then get the expensive body!