They were probably thinking that especially on ultra-wides, there is not that much use for filters, so it doesn't matter that much. Graduated NDs are usually bigger than 82mm, so that doesn't matter so much. Regular NDs, at least the ones I use, are also bigger than 82mm, and use the same (square/rectangular) mounting system that the grads use. Color correction filtering is better done with the camera, and that costs less, is lighter and is hard to misplace. (But not impossible!) So the only circular filter really left is a polarizer, which many people won't use on ultra-wides. Sure it is probably fine at 28mm and certainly is at 35, even by the most stringent standards - but then you probably have that covered a different way.
Do a search on the third party lens forum - there are some really nice results shown by members using the 17-35/f4. Maybe some with the 16-28 too, but I'm sure of the 17-35.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!