Cardio is not a waste of time in my opinion.
But I have read elsewhere that there are two schools of thought on this- I even met the other one in the form of a personal fitness instructor who offered me a free session. To my amazement he claimed to have lost quite a few stones in weight by only lifing weights- he said to me to lift heavy weights and do this for 15 reps, 3-10 sets, less then 60 seconds break etc, then if I felt like it, 15 minx max of cardio- his idea of cardio was a slow jog on a machine.
Now I think that was daft- I understand the bases of weight lifting in the area of weight loss, but no cardio? I am now 99% sure that suppliments where used in his weight loss!
Cardio for me (which has become a bit of an addictive obsession and am trying to cut back or risk my legs!) has given me boundless energy! I rarely seem to get mentally tired and tend to find injuries or strains put me out over any energy loss. Day to day I seem to amount to the same level- if not more, as the energy levels of my 5 year old niece.
I also rarely get ill- in fact this last week I think I should have been ill- my glands were very swollen and I had a blocked nose and chapped lips. Yet I still had the energy to workout, speak and see people as per normal. I amount that to cardio keeping my body healthier then it otherwise would be- infections don't seem to get as bad as they otherwise would, and I recover from them pretty fast when I do get them.
I also assume that my heart is in good condition. From what I understand the heart is a muscle. It gets stronger by working it out.
Then there are the endorphines and dopermine (please excuse my spelling, I am not an academic chic!)- not just the chemical stuff, but also the awareness that your doing something that was quite challenging and yet you overcame it/met your goal/did something amazing. It feels good, it boosts your confidence and after a rubbish day at home/work/college...etc it can feel so welcoming to have something like exercise to remind you that you are infact good at something, you can do something great and also can buy yourself peace and quiet for however long you workout for.
Its almost like a good nights sleep when your done, your outlook on things can change considerably, a whole new perspective. Some of my best design ideas have come from mental images I conjure up when halfway through a workout.