It's not a matter of hunters not stacking well. It's that Heroic instance content scales beyond the tanking capabilities of a hunter pet.
Yes, you can over-gear the content significantly, but PvE as a multiboxer is no fun if you have to outgear it in order for it to be even viable. The parallel to 5xShamans is interesting and relevant. 5xShamans can kill heroic content once they're overgeared, but the real allure is that they're strong at PvP. 4x-5xHunters are not strong for PvP outside of some very narrowly limited applications.
It's just a very hard road to travel and the end result is you're mediocre (at best) at just about any aspect of the game once you hit end-game content. Ask anyone that has played the comp and they'll say they had a fun time leveling them, but there's a reason why the few of us who played the comp seriously prior to WotLK have all dropped it in the last year...
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