It depends entirely on the people involved. Opposites can be good if they complement each other, rather than clash. People who are similar may be more forgiving of each others' foibles because they share them; however, two strong personalities with different ways of doing the same thing could be trouble (likewise two absent-minded personalities who forget to pay bills). The convenience argument seems more directed towards sex or short-term fulfillment, like Matt said, than anything lasting; it does not preclude growth, however.