Agreed. I know the majority of styles will (and already) do it. By default, XF2 should have it, and it will establish a base framework for all customer styles to have it too.
There are so many sites that utilize the sticky navigation these days because they see the benefit in it. Causing visitors to scroll up to get to another part of your site is unnecessary when options are out there to easily implement it.
In XF 1 we've done this as well as others in frameworks and it always feels "tacked" on especially when we have to provide workarounds for anchor links in posts, which do not always carry over to conversations and profile posts.