As to your question of why .66 is used, I have no idea. I'm the type that, if I'm interested in something, I'll research it to death to understand why it's done that way too. Which should tell you that I've really had no interest in being a proponent of ABB.
When I first read HoH and learned of M, I thought it was the greatest idea ever and would be far more useful and accurate than relying on #bb's. I started thinking in terms of green/yellow/orange/red zones, and when HEM added the M stat I immediately added it to my HUD and color coded it accordingly so that I could immediately recognize mine and my opponents' M at a glance.
After awhile, I fell back to bb's.
Why? Because even though M (and ABB) do more accurately affect your stack lifetime, virtually all training and feedback I'm accustomed to (books, videos, forum HA's, TV/pro commentary, etc.) still refer to bb's, and tend to ingrain decision points around bb's. Almost no body of work that I've come across except Harrington seems to use M these days. All the MTT videos I watch still talke about bb. They never mention M in the WSOP
and other live game broadcasts -- it's always bb. The constant juggling of M vs bb became tiresome to me, and I preferred the easy/lazy route of speaking the same language as most everyone else.
I suppose for beginner/intermediate players that are in the process of taking their game to the next level, or maybe some obsessive students of the game who like a single stat that wraps everything up nicely for their autopilot decisions, a tool like M or ABB is useful.
However, I think the reason that more advanced players don't seem to put as much emphasis on it, and still refer to decisions and stacks in terms of bb's, is a because as you get better at poker I think you learn to just automatically factor antes into your decision. If I have 10bbs with no antes, I subconsciously make different decisions than if I have 10bbs with an ante. This allows me to consider antes while speaking the same language of bb's as most everyone else. Same if I'm playing a turbo structure vs a normal/slow structure. It's one of those adjustments that good poker players have to make on the fly.
I guess the best comparison I can come up with would be the metric system -- while I feel that it's a far more logical and intuitive system of measure than the Imperial system we use in the US, and would love for it to have taken off here, unfortunately it didn't. If you try to insist on using the metric system in your daily activities in the US, it becomes really awkward and tiresome because you're doing the conversions constantly both for yourself and for others you may be interacting with (unless you happen to live in the military or scientific world where metric is the more common system).