I do agree that cardio is important!
No doubt about that, but perhaps I should be more clear with why I made the recommendations I did...
I don't think you'll find many trainers or serious bodybuilders who will tell you to do a lot of cardio when your goal is to simply gain lean mass. There's a trade off in that which makes it counter productive to gaining mass. Can you do a lot of cardio AND gain mass? Yes, but it's much harder because you have to consume more calories and more protein to offset what you burn and your gains may take much longer to achieve. Talk to guys who do bodybuilding competitions and when they're trying to gain, most of them will tell you they limit their cardio in the gain stages.
Here's an example of what I'm talking about:
If your goal is to gain lean mass and especially if you're a hard gainer (like me), you should do fine to workout with weights 5x a week (remember more muscle burns more calories and in the process also impacts your metabolism). If you feel you should add cardio, then 2x a week should be sufficient and it should be limited to 20 to 45 minute sessions. Also, remember that whatever routine you choose, your body needs rest and recovery time.
I do agree with MSUBioNerd that for individuals who want to gain lean mass, shorter bursts of cardio is the way to go. Here's a great way to do cardio on a treadmill if your goal is gaining lean mass: Try doing 2 minutes at a moderate pace, then 1 minute at a fast pace, 2 minutes at a moderate pace, then 1 minute at a faster pace, and continue to alternate for the total number of minutes you chose to run.
But take everything with a grain of salt...
You have to figure out what works for you! As we grow older, cardio becomes even more important, this is not to say that it's not important when you're young, it's just that our body does change as we get older. Also, to insinuate or assume that your body will not gain any health benefits if all you do is build muscle is inaccurate. Remember health is in it's most general terms about eating right and staying active... however that active part turns out for you.
But this does lead me to one pet peeve of mine when it comes to weight training...
It seems like a lot of guys will pick a muscle and just focus on that, like biceps, or pecs and neglect others. I see soo many guys who look like "hulk" up top, but have chicken legs. Or they have a huge chest, but no back muscles.
Symmetry is the key word. It's important to develop your muscles in relationship to opposing muscle groups to avoid long term affects on your posture and on your joints and tendons.
But finding out what works for you is part of the fun.
So, good luck with whatever plan you choose. I know weight training can be daunting in the beginning. There is soo much info out there. Some of it is good, some of it is bad. And just because someone may be built like Arnold, doesn't mean they know the perfect routines.
Just do your research and discover what works best for you. That way you'll appreciate your efforts even more. ;)