Its not DISPEL!!! /drool, its "I get 3 dispels in 15 minutes. Is that worth only getting 2 guaranteed TAs in 15 minutes?" That is the basis of opportunity cost.
It's really not. If we could create an apples to apples comparison like that this debate would be easy and nice, but that's not possible. When you put a merit into aura steal you are not decreasing a timer. You are giving yourself a functionality. If you're riding timers constantly and mobs have buffs up constantly then yes your scenario comes into play. But only paying attention to that scenario and ignoring all the other ones is being incredibly biased. You have
to consider every situation your thief is going to be in. Putting one merit into Aura Steal gives you the ability to dispel when solo. It gives you the ability to take off an evasion boost that the mages weren't going to dispel that decreased the dot of all 6 melees hitting the mob. Etc. Its effectiveness doesn't become completely useless if you don't use it less than every 7.5 minutes.
If we do your comparison and Aura Steal wins, then it's clear who the victor is. But if we do your comparison and Aura Steal loses, it's still not clear which is better.
I agree that that isn't the whole answer, however, in order to make an informed decision you do need to be aware of what the maximum potential is for both senarios. The maximum cost of having dispel is 1 triple attack guaranteed. The cost of having 1 more triple attack guaranteed from 2 to 3 is 3 dispels. And that is the maximum potential of both of these options. You have to establish the cost of one option over the other before you can actually make a fact based decision.
Then you get into things like...
am I soloing/low manning more than group activites? [Yes, well, extra TAs are useless to me since I won't be able to SA/TA + WS ever, Aura steal it is.], [No, I do all big group stuff, I have 6 rdms that come regularly, AS isn't as useful, I think I will go AC, or No, I do all big group stuff, but having my own dispel will still be useful]. Be aware that I am being kind of silly with one of those HYPOTHETICAL thought processes.
do I fight mobs that actually buff with regularity? [Yes, some points towards meriting AS over AC], [No, well AS is completely useless then as I won't be able to dispel anything, AC, here we go!!]
Here is a list of the things I have been fighting lately and how useful AS has been.
Orthus - basically that annoying flame spikes every time he uses, which is often, AS is useful for this
Bennu - Evasion boost, Feint is more useful than AS for getting rid of it, and on a smaller timer. AS still useful, but dulled because of feint.
Blanga - Atk bonus, meh, still hits like a truck without it, but I have shadows anyway.
Durinn - No buffs that I know of
Buhkis - No buffs that I know of
Briarius - No buffs that I know of
Glavoid - No real sure, don't think there is anything I can dispel
Chloris - Regen I guess, maybe something else
Dragua - I am sure there are buffs, but I haven't fought it in a while so I don't know what.
Resheph - No buffs
Khimaera/Hydra - sure lots of buffs, AS is useful
This is why I think you guys are making a mountain out of a mole hill with AS. Sure it is nice having a dispel handy, but at least 50% of my current target mobs can't even buff themselves. It just isn't that useful. I keep it because I do find utility in it, and having it is worth losing 1 TA in fifteen minutes to me, but it isn't as game breaking as it is being made out to be.