1. Whatever way you look at it, it will be an extra barrier of entry (cost, legal hassle, more complex chip design) for any new ASIC manufacturer trying to enter the market. That counters free competition and thus decentralization.

2. Why would you want to put yourself in the central spot of the big decider on who gets access to the technology (and therefore the whole mining game) and who doesn't. You're not afraid of NSA knocking on your door to politely hand you their blacklist? You don't think this counters all the years of hard work that went into Bitcoin exactly to avoid any such central points of authority?

P.S. I'm not decided yet on being for or against a HF to ban AsicBoost myself, nor does my opinion count for much. But I think I do see real problems, like the above.