Currently today to pull freq() from a docsEnum, you have to pass a boolean 'needsFreqs=true'. If the field omitsTF, then it returns null (and you need to call it again, with 'needsFreqs=false', in which case you are not supposed to call freq().
We fixed this in D&PEnum in 4230: as you can tell from the fieldinfos whether they are there or not, there is no need to return null, it makes consumer code complicated.
So this issue is just to have docs(Bits, reuse) which calls docs(Bits, reuse, FLAG_FREQS) by default. if they arent there, the docsenum returns 1 for freq().
So calling docs(Bits, reuse, 0) is just an optimization hint to the codec that you never need them (same as the payload/offset flags for docsAndPositions)