The "prototypical" third baseman is no longer typical. Third base is no longer a power position now that the steroid era is drawing to a close. Fielding ability there is much more important now than it has been in the past 20+ years. You could argue that it has always been important. Mark Reynolds, for example, is one of the better and more powerful hitters at third base. Yet, he is one of the least valuable because his fielding is so bad. Third basemen no longer have to be the big lumbering slugger that scouts and managers and fans envisioned the past two decades.
As far as projecting Cowart's potential (mike lowell), We are still a little ahead of the game on that. the highest level he has played is high A ball, and many young players look good there. Callaspo is one of the best fielding third baseman at the major league level. To say Cowart is better than him may be a little offbase. A current, effective major league player is more valuable than just about any prospect. You just never know what you are going to get down the line. Right now, we know what we have with callaspo. Cowart, we don't.
The old saying goes, "it's better to have a bird in the hand than two in the bush." We have a pretty good bird in our hands. For very cheap.