I didn't see DXIL support as being particularly relevant to 1.9 because it would be the barest minimum, but I realise it's meaningful as a milestone to have a test pass. Are there other reasons I'm not aware of?
Fundamentally, we'd like development to happen upstream as much as possible. There are situations where developing/prototyping something on a branch first makes sense, and this is certainly one of them, but there are costs to maintaining branches. The most obvious of those is perhaps the time spent rebasing to account for upstream changes, but you can also think about e.g. the branch diverging from what's acceptable upstream, people spending time debugging issues that are already fixed on the branch, different people independently writing the same patches, and so on. I.e., we'd simply like to minimise the set of changes that's not upstream as soon as possible. And to give some sense of the kind of upstreaming rate I'd look for, I think for about 80 percent or so of this branch, about 20 patches a week on average should be achievable.