First of all, GoM still works on other spells besides quick heals. This means that there is less than the pure mathematical chance that follows, because the GoM random number generator is spreading its chances over other spells as well.
Assuming you have maxxed both GoM and AH, which is the only case where it will potentially block GoM, you will get a 1 in 10 chance of AH firing. If somehow the planets were in alignment on the instant you got that AH proc, there is another 1 in 10 chance that a GoM proc went off at that exact same time.... that's a 1% chance that both procced on the same quick heal cast....
They're bitching that in less than 1 in 100 quick heals they will get an AH proc that blocked a GoM proc. This means when you cast 100 quick heals, 10 of them are AH procs, 10 of them would have been GoM procs, and the other 80 are normal. With this "bug," 1 of those 10 GoM procs ends up being an AH proc instead. You can't even feel 1%.... unless you're one of those greedy little misers with 100 pennies for a dollar, and can "tell" when one cent is missing.

With both AAs, you get 20% chance of extra goodness (10% AH + 10% GoM). You get more healing power thanks to AH, and you get mana saved thanks to GoM. For those holding out on AH because of its 1% chance to block 1 in 10 GoM procs, you lose out on all that healing only to be satisfied that that super rare proc remains a GoM proc. Someone please enlighten me as to why I should sympathize with this needless whining?