Yeah, maintaining 3-5 stack of Potent Flames is incredibly important for the long term dps gains you get from Pyroblast. Instead of spamming two pyroblasts back to back you should try scorching or fireballing again before using that second pyroblast charge. It's kinda like half camping Pyroblast procs and if they both crit its gg. Inferno blast, fireball pyro again and you're sitting pretty with high stacks and 1 1/2 procs ready to go.
I can totally understanding going back to back with at least an inferno blast thrown inbetween or if an add needs to die that very second, otherwise I feel it's a waste of a proc that just gets used on a fireball and not another pyro after that getting an extra tick from the amount of haste you're getting from the high buff stack.
On our first H Garrosh kill the only reason I managed to catch back up to multi dotters was maintaing a high stack of Potent Flames as much as possible. Weaving in scorches was critical to this with lower ilvls. (also gg warriors were on point with skull-banners)
http://www.youtube.com/watch?v=UwCWRoxRPvo