You may.
I train for primarily strength and mass. I use CV not only to help keep healthy, fit, drop body fat and make me feel alert but also to make this mass and strength functional.
The heart is a muscle and should be trained just as hard as any other. There's no point having big strong legs if your heart can't support supplying them with blood to move. It's all about balance but also proportioning.
I see.. Good point about the heart muscle, i agree. I trian for a sport, so no need for me to put on body mass as such because the weight i am is perfect for the weight group i compete in.. Kinda explains my need for good CV levels.