NothingIsRealButTheGirl wrote:
Changing the definition of opacity midstream seems a little kludgey to me, especially since that behavior is probably not well documented and has to be intuited all touchy-feely-like. But whatev.

I completely agree and will further suggest that AE's manner of doing things (as I assume that all the other same-name blend modes have the same results, even with opacity applied) is also a hack - an 'add' which is calculating around its own blend so as to get a result which isn't actually true to all the other definitions.  But clearly the users are happy, so I'll +1 the whatev .