Personally, I find some of the truly tangential stories to be among the best. Nemesis, Outcast Dead really caught my attention.
Beyond that, I'm among those that wish they'd just get on with it already.
Tell the main story, then leave HH around as an imprint to tell 30k stories. They've shown plenty of willingness to go back and forward in the chronology vs. the release cycle and it doesn't seem to be bothering anyone. I, for one, am actually looking forward more to what they do AFTER the siege of terra. There's easily a dozen novels that can be written about the events after the battle aboard the Vengeful Spirit. You've got the flight of the CSMs, the events of the second founding, the "departures" of the loyalist primarchs, and so on.
Meanwhile, those nice side stories can still be written afterwards. We'll still buy em, I'm sure.