Sometimes this is a good idea. Many of the best
marriages I've seen lately have been between, for instance, an
introvert and an
extrovert, or an
athlete and a
domestic engineer. The personality traits that one person lacks and
envies are brought out by his/her partner, to the benefit of both.
However, if the envy and desire to change does not exist, problems will eventually result. If one half of a relationship is very involved in charitable activities and the other wishes to be, then both people are happy. But if the other partner thinks charitable activities are a waste of time and resources, there's going to be problems, and they'll only get worse as the relationship gets closer.
It depends on your priorities, on what's important to each of you. If you have opposite traits and qualities, but identical ambitions and goals, you should do great. If you have opposite ambitions and goals, however, then one day you'll find yourselves living your lives in opposite directions, and your relationship is going to rip right down the middle.