Shamrock started out as a pro-wrestler years before he got into fighting. When he started with WWE he picked it up again very fast. He actually turned into a really good worker and adapted to the WWE style, meaning he did more than just shoot-style stuff.
Severn didn't have the right kind of personality for WWE and I don't think they really had any idea what to do with him. Tank Abbott had an amateur background but made his name in UFC brawling. He never learned to work at all and was a victim of Vince Russo 's wackiness in WCW.
There is no downside to it whatsoever. If you make one star out of it, you've come ahead. In a good season, they'll make a handful of stars. Despite what some hardcore fans might say, a card with five stars on it will do better business than card with zero stars on it, even if those zero stars are the best fighters in the world.
Picture Quotes to Inspire and Delight