After a little more thought, and ignoring the current mechanics for a moment, and ignoring the instant cast situation, what I would like these huge macros to do (all on the client side of course) is:
Scanning the huge macro from top to bottom, cast or queue the first available ability. Keep the queued ability current with respect to this huge macro at all times.
Perhaps many of the constructs one finds in these huge macros are just a way of trying to implement that statement with the current game mechanics. The number of available abilities can be overwhelming, as others have also stated. The second sentence in that desire leads to the clicking of the macro button (or key) as fast as possible; (it would be so nice if one of these "spare" processor cores could handle that boring task). Notice: no scanning from bottom to top for the ability to queue.
Remember, "first available ability", as mentioned just now above, takes into account everything (from the client's point of view of course), not just spell reuse timers. (range, target type, available power(mana), etc.)
-Leucippus