I think that we should not mess with evolution. I can understand one's curiosity but at the same time once you alter an animal that has reproductive capabilities, then you have basically re-engineered the species from what nature has pre-programmed for that particular species.
Altering animals to meet our needs isnt something new, however doing it on the molecular level is as we've only been at this level for half a century. Only recently were scientists really able to begin to understand it, how it works, and what parts affect development and physical characteristics.
Look at dog breeds, we have been breeding and crossing breeds of dogs for centuries to obtain what we feel is the ideal dog for a given situation. We culled the dogs that did not have the desirable traits, and those that had the best of the desirable traits went on to become prize stud dogs and bitches. When we became successful at that, we understood the basics of genetically altering animals, but no certain traits were gauranteed.
These days at the embryonic levels, we can almost gaurantee certain traits. We have cloned animals, we have made great strides in medical research using stem cells, but we have to draw the line somewhere and say hey - when it comes to this we are messing with nature which can be extremely dangerous.