First of all, let me say that I hate cardio and I do not jog or bike. The way that I actually get a cardio-type workout is by walking and a kind of circuit type training when I work out. That said, I recently had a forum poster make the following comment and I see this quite often in the health press:
"As for my lifestyle,I should tell you that I know what you mean. There is no way I overtrain myself, because I use a HIT technique I "invented" I train once a week, one week upper body next week lower body. I set to failure for each muscle for example , squats for legs, military presses for shoulders etc etc It take me 45 minutes at the most each workout. No "cardio" at all, total waist of time, proved by me." 
So is this gentleman correct? Can one just exercise once per week as long it includes HIIT (interval training) and some weight lifting?
Now I am not trying to be disrespectful, but there are MANY reasons that guys should should not only include cardio but also WANT to do it. Remember: I hate traditional cardio and yet I am saying this! So before you throw out cardio, consider these great reasons to hit the pavement or visit the gym:
1. Baseline Nitric Oxide. The experts with Berkeley Nitric Oxide Test Strips have pointed out - and they work with a lot of highly trained individuals - that athletes tend to have elevated baseline nitric oxide levels. Now this is every guy's fantasy! Who doesn't want nice, beefy nitric oxide levels day and night? (Just don't overtrain or no amount of nitric oxide will do you any good.
CAUTION: I use the word "cardio" above, but it is actually moderate exercise that confers the most benefit with exercise. Moderate exercise is defined based on VO2Max and/or heart rate and is bascially the kind of intensity you would get from brisk walking on the low end to jogging on the high end. So, if you don't like words like "cardio" or "aerobics," just substitue the term "moderate exercise" instead and you'll be in good company. Also, I want to be clear that there are HUGE benefits from interval and weight training, which the original poster mentioned. I do these multiple times per week, but that is a subject for a separate page.
2. Post-Exercise Nitric Oxide. Suppose just for a minute that you don't exercise enough to raise your baseline nitric oxide levels like an athlete. Well, the studies show that anyone who exercises gets a nice nitric oxide boost for 2-3 hours after exercise. So the ideal would be for a guy to go for a brisk 20-minute walk 3 times a day after breakfast, lunch and dinner. He would then have improved nitric oxide levels almost the entire day.
Now contrast that with someone who exericises just once or twice per week: he has to come up with some other way to boost his NO levels. Now, if you're young, you probably won't notice, since your arteries are still relatively clear. But anyone with a decent amount of atherosclerosis or aging can really use any boost in nitric oxide.
3. Triglyceride Removal. One of your big goals as a male, who wants to occasionally do the wild thing whenever his honey is so inclined, is to have a decent erection. Well, that ain't happening if your arteries are full of plaque and one of the primary reason for plaque build up is elevated triglyceride levels. After you eat a meal with carbohydrates, your triglycerides will surge. However, you can drop these levels by around 25% or even a little more simply by exercising. Again, a little walk or exercise of some sort is a GREAT idea for us men.
4. Brain Testosterone. Researchers recently found a rather astonishing fact: exercise works it's brain magic by increasing testosterone levels behind the blood-brain barrier. If you've ever felt fantastic after exercising, this is one of the reasons (along with a nice increase in endorphins). Exercise and activity are FANTASTIC for the brain. Study after study after study has shown this: exercise can greatly help with depression for example.
5. Weight Management. Do you have to exercise to keep the weight off? Of course not. It's mostly about what you're shoving down your gullet of course. But consider this: the average adult that is putting on the pounds usually will again, at most, ten pounds of fat in a year. That is, if you do the math, around 35,000 extra calories per year.
Now divide those 35,000 calories by 365 days in a year and what doyou get? Only 100 calories! Thus, those who exericse are naturally burning the extra calories that we so easily consume in modern societies since food is so abundant and plentiful generally. Exercising once per week is going to require you to have steely discipline when it comes to food consumption, something most of us do not have with a busy, stressful lifestyle.
6. Sitting = Smoking. If you're not sitting, what are you doing? Smoking! "Wait, I don't smoke Lee!" Yes, you do if you are sitting. Studies show that sitting is just as hard on the body as smoking, something I discuss in my link on Sitting: A Bad Way to Die. Some of the issue has to do with the fact that lipase, a digestive enzyme, is ground zero for lipid metabolism and transport. Your lipase pools and does not circulate unless you are moving.
SUMMARY: Again, I mean no disrespect to the above poster. He may do fine exercising once per week if he is very active doing yardwork and things around the house with family and friends. If he moves a lot, then you have a lot of natural protection built in. But, even so, there are huge benefits to "cardio," or better termed "moderate exercise." For additional reasons, see my page on Sexual Function and Exercise.