Excellent post, Exile. Good points raised. If you KNOW it is carcinogenic, why willingly expose yourself to it, when you don't have to?
Another thought to consider: cardio is all about breathing. You breath hard as you work hard, the O2 gets taken into the blood and all around your body and this plays a huge role in how well your body performs. If this was cut off for even a few minutes, irreversible damage and even death can occur.
When you smoke, you directly interfere with this systsm. Read that again: YOU DIRECTLY INTERFERE WITH THIS SYSTEM. Your body's ability to breathe deeply, absorb the O2 into the blood, and your different body organ's ability take the O2 and use it is greatly diminished. If your smoking level is low enough you may not think you will notice a difference, but the truth is, you are interfereing with a crucial bodily function. All performance will suffer because of this.