Is that something that will hold true if we add more codecs (or change existing ones)?
Not necessarily but it's been used to distinguish the WMV decoder from others already in some other part of the code. There's only tests for the H264 and the WMV decoder but it seems to me that not generating timestamps/duration is probably the special case here.
I'm even surprised we need to strip them, I suspect it is specific to the IMFTransform interface of the WMV decoder as otherwise I don't see how things could work without timestamps otherwise.