You are never wasting your time with weights, and if you want to look good after slimming down then weights are more important than cardio if anything (and it's not enough to just do your arms).
It's difficult to put on muscle mass whilst in a caloric deficit, but you can make good gains with your strength and more importantly maintain your current muscle mass if you keep lifting whilst cutting. You could see some newbie gains, but they'll always be limited whilst in a deficit.