Before going into too much detail on the thread, consider what you actually need out of a fancy directory iterator. Offhand, I really can't think of many places where that even *happens* in MediaWiki... maybe when purging thumbnails?
I count 10 instances of opendir() exactly in trunk.
takes more effort than just using things built into PHP (there's at least 3 or for directory iterator systems already, including the opendir() stuff, the object-oriented variant in the Dir class, and the SPL iterators).
The main part of my push for this is that it isn't a new feature, but an easier way to read and write code. Myself, at least, readdir(), opendir(), etc is very arcane and confusing (same reason I don't use fopen, etc). RDI and SPL are even more confusing. I'm just going from ym own experience here, but I'm just thinking that it would be a good addition.
-X!