The big difference here is that while of course heals aren't affected by regen debuffs, it is only a chance for a heal which means it isn't consistent. Regen is a reliable, known value - even after factoring in debuffs you can see exactly what your regen rate is at any given moment and you know that you will get that much HP/sec back. The heal proc, while mathematically superior over time, isn't always going to give you that amount of HP/sec. Sometimes it'll proc more often, and sometimes it won't proc at all for a minute or two while your regen is returning hundreds of HP.
Re: tanking Hamidon, Hami does -regen and -heal so it won't make as much difference as you'd think.