Bluemagic, I'm afraid your math is wrong here. While the Bloodlust Brooch is active, it adds 278 AP IN ADDITION TO its base 72 AP. So the calculation would be:
72 x 120
+
278 x 20
= 14200 combined AP over the two mins.
So, technically, the Bloodlust Brooch still provides more AP.
Another way to think about it is this, the Bloodlust Brooch (if you constantly activate it as soon as it's cooldown is up) will provide an average of 118.3 AP. While the Stone will provide a constant 108 AP.
Of course, no one can actually remember to always keep Bloodlust Brooch active 100% of the time it is available, so in actual usage, I think the Stone will still probably do more for your DPS over the course of an evening...
72 x 120
+
278 x 20
= 14200 combined AP over the two mins.
So, technically, the Bloodlust Brooch still provides more AP.
Another way to think about it is this, the Bloodlust Brooch (if you constantly activate it as soon as it's cooldown is up) will provide an average of 118.3 AP. While the Stone will provide a constant 108 AP.
Of course, no one can actually remember to always keep Bloodlust Brooch active 100% of the time it is available, so in actual usage, I think the Stone will still probably do more for your DPS over the course of an evening...
Your math is fine, but your conclusion isn't. Bloodlust Brooch will be active probably about 50% of the time through trash kills. All the time you are stopped for mana, bios, rezzes, rebuffs, etc. ad infinidum your BB will be on CD, getting ready for you to use again. It gives me 72+278 = 350 AP most of the time through trash. Only in a boss fight will it lose this luster. At worst, using it at every chance would result in the effect 20/120 seconds. Starting a boss fight (once tank has good aggro) with it results more often in it being used 2-3 times. (ex. 20 seconds on. 100 seconds off. 20 seconds on. 20 seconds off. end fight = 40/160 seconds -> up 1/4 time.) Thats still pretty good. And if you have a trinket that give you massive AP gains and you are never using it b/c you "forget" you should shoot yourself and quit WoW (not in that order).