I think that the point of replicating "PHB-style" rules in AU, GlassJaw, was to make the book a standalone. There have been a number of posters on these boards (no one I know personally) who run AU and don't own a PHB.
As for borrowing: I've mined AU extensively for my FR campaign, and probably will do so for my upcoming Midnight campaign. Things I use in FR:
1) Most of the magic system, including the simple/complex/exotic spell categories, the spell templates, diminished and heightened spell versions, and the system of lading spells to create effects rather than using the D&D metamagic feats. This has worked out spectacularly.
In general, I use AU versions of spells instead of D&D one wherever both exist, and ban several D&D spells that I feel are flat-out too good when compared with AU equivalents (magic missile, fireball, resurrection).
2) The magister class, which is called the "mage" (since FR has a very campaign-specific definition of "Magister") and replaces the sorc and wizard IMC. I find this class much more appealing than the D&D sorc and wizard classes. I've added many of the iconic D&D spells to the magister class list (divided into simple/complex/exotic categories based on power and assumed rarity, as suggested by Monte in the AU DM's Screen and Player's Guide), and as a compensating factor, I've lowered magister HD to d4 and limited them to gaining 10 new simple spells and 5 new complex spells (or exotic spells, if these are treated as complex spells due to possession of a template) every level. The flavor of FR is preserved by requiring magisters to keep spellbooks, except that they do not need to use those spellbooks to ready spells; instead, the spellbooks contain all their complex and exotic spells known.
3) The warmain and unfettered as "alt.fighters." Fighters can choose the heavy (warmain), light (unfettered), or skilful (D&D fighter) paths. These are treated as paths for the same class; warmain/fighter, unfettered/fighter, or warmain/unfettered multiclassing is prohibited just as invoker/enchanter multiclassing would be.
4) Truenames. There is already a precedent for these existing in FR, and I think they're cool to boot!
5) The item creation system, including the different feats and the spell-based multipliers; this is just how D&D should have done it from the get-go, IMHO!
Mind you, I think that all of the above are easily folded into a standard D&D (FR) campaign. The magister blows away the wiz and sorc as a spellcaster class concept, IMHO, the warmain and unfettered allow good modeling of the armored knight and swashbuckler concepts, and the magic system is full of the adaptability, flexibility, and variety that characterizes FR magic.
I find the races, the totem warriors, and the akashic a bit unusual to tie into a setting like FR, though I think the greenbond is an easy fit; the problem with this latter class, I think, is that it's flat-out inferior in power to either the cleric or the druid, which means it's just unusable in conjunction with those classes. I'm planning on using the greenbond (and NO druids, and only evil NPC clerics) in my upcoming Midnight game, though.
I'd really give the AU magic system another look, though; if I were writing a 4th edition, it's the one I'd go with.