If you want to learn about HIIT training, you have to look outside of bodybuilding to sports like track, swimming, and cycling.
The main problem with HIIT training is that it very taxing to recover from.
The best part about being a bodybuilder is that you do HIIT training every day in the gym when you train.
Your heart needs two things, HIIT train and LISS training(Or perhaps optimally, moderate intensity steady state). If you ONLY give it HIIT training, you will suffer from a lack of blood volume and overly high hematocrit, you will also not lower your blood pressure adequately. As one semi-famous cardiologist (he did the physical fitness exams for the secret service) once told me: "Between intervals, the heart is in 'free fall' and isn't contracting hard, that is why you need steady state to lower blood pressure, lowering blood pressure is about more than your heart, it is about your whole cardiovascular system." I'm paraphrasing but that is about exactly what he said. I've spoken to many other doctors, one in particular who had a deep understanding of the heart and blood pressure (an anesthesiologist) who agreed.
Anyhow, all I need is a lot of personal experience and a blood pressure cuff, I can tell that 20min per day of steady-state works better than anything else I do to keep my blood pressure down with the absolute minimal inroads into my recovery.
Would my heart be in even better shape if I did additional HIIT cardio? I'm sure it would, HIIT cardio is extremely beneficial up to about 1 hour per day, however, I hate cardio and I have no interested in limiting my ability to recover (and thus limit muscle gained) for that tinny bit of extra cardio function.
If you understand what HIIT training is (Anaerobic threshold training with the name ripped off from HIT training), then you know that we all have very high anaerobic thresholds from weight training, HIIT training isn't what most bodybuilders need the most, in fact it can be counterproductive in a growth or maintaince phase.
edit: most people I see in the gym do not train hard enough to be any form of anaerobic threshold training, but I do, and a handful of others do. I would assume most people here do but I might be wrong.