i used to think the same way,im a contractor also but i finally added cardio in this past year and i look and feel better..
plus it is good for your heart.
my gains have not slowed,im just not as big and fat as i was.
Then your one of few lucky few that can get away with it or your
not into heavy labor (not meant to be smart ass). I run a
landscape business and sometimes put in 10 12 hoiurs a day, and
because this am always overtraining, and on busy weeks lose alot
of size and strength. Problem with this kind of work
the lack of time you have to get the right amount of food it takes
to keep your hard earn muscle and the fact your using your muscles
to push, pull and lift all day. So for me cardio would definitly be out
of the question.
I also think that this is something the body will not get accustomed
to if you add cardio if anything you will drag your self down to
the point where getting out of bed would be a chore. Believe me,
I've been doing this for 35 years.