After a certain time when training, cortisol is released whatever exercises you're doing. HIIT cardio improves explosive strength and CV fitness over a short time spent actually doing the exercise, limiting cortisol release (I assume you know this as you mention sprints). You also get more mobility of joints, strengthening them and like I said above, better circulation throughout the body I'm aware that weight training has it's CV benefits, and yes more muscle mass will increase basal metabolic rate. But ruling out cardio is a bit of a waste of time; adding contrasting exercises eg cardio into strength training regimes will only help improve recovery time and performance. I study physiology and all the rest of this malarkey at uni lol I'm not knocking strength training, I actually prefer it But I'd always vouch for including cardio in a strength training programme even if it isn't a great deal. The amount it should be done depends from person to person of course