I don't have a complete collection yet, but I do buy ships just for that one card I need for a killer list.
As for balance in the collection, you can always artificially limit your collection whenever you choose. Say, for example, you'll only play with waves 1-3 for a month. Or say OT ships only. I'm going to keep plugging away at this game for as long as possible. I don't see an end in sight.