Whether or not it's a good idea to incorporate some kind of cardio depends on what you want to achieve - fat loss? strength gains? larger muscles? better performance on the soccer field? aesthetic improvement? all round fitness?

Being fit means different things to different people. Personally if I can't run 10k in under 50 minutes I don't think I'm fit enough (but that's not my only threshold for fitness), but other people will have their own idea of what they consider to be fit.