20-30min. of cardio i doubt will affect you that much (in the negative aspect of losing muscle or strength). I'd have to agree that benefits far outweigh the suppossed consequences.
i hate listening to some people whine about how there gonna burn away all their muscle. There a big difference in running 30-40 mins 3-4 times per week and running a damn marathon. My last cycle i didnt do cardio for 10 full weeks. FELT SO FAT, cardio hardens me up so well and gets rid of all my water.