[Akryn]

Back when 2.0 came out and they removed the ability for scripts to cast arbitrarily, they justified the [form] condition by saying that stances/forms/stealth change the player's ability to cast certain things, and it made sense for macros to be able to do different things depending on which one was active.

So that's Bliz's answer: "buffs" that don't affect which spells you're allowed to cast can't affect macro cast decisions (for the same reason that active debuffs on party members/targets can't affect spell/targeting decisions).

Edit: I just noticed that you actually did ask to be able to be able to check target debuffs. Abuse of that ability is 95% of the reason that the current macro system was put in place, replacing CastSpellByName and TargetUnit as player-usable APIs -- so I highly doubt you'll ever see that come back again.