Come to think of it - another reason why I've seen it so often is that it is often used in the titles and headers of engineering/scientific papers, and I've seen a fair few conference publications with collected papers in because that's what my father did.
This is an example of the type of usage:http://cfa-www.harvard.edu/cfa/oir/MMT/mmt/fo...
Please note, Alexander, that the author has capitalized WRT because the words would have been capitalized in a document heading had they been written in full.
Having said all that, I just checked my Oxford reference shelf 'Dictionary of Abbreviations' and it says:
w.r.t. with respect to !!
So maybe some more headgear needs to be munched - it is standard english with full stops in, but modern usage is stripping them out.
Maybe @MissG could chip in and tell us the rules about using punctuation marks with abbreviations.