Airfilter oil is pretty cheap, and the filter is made to flow this way.
Youd' be surprised how much crap it catches when you clean it.

But on a stock DTR, nah it's not going to impact fueling

Some dirt tracks, like ashdown mx track, in the summer is like a dust storm. That's going to go through the filter if it's unoiled.

But for general road driving, I'd expect very little.

The other one is when they lay down new tarmac, they destroy the road first then grit it, that kicks up loads of dust which is unlikely to be caught by the filter.

For reliability sake, I'd oil the filter up.

If your engine is modified, relability goes out the window anyway.

Depends if you want the maximum life out of your engine, or if you don't mind doing a rebuild every once in a while.