The benefit would be the ability to cherry pick only the most desktop-beneficial features rooted in non-desktop domains while also leaving behind the rest. One example would be the window server and compositor — on Linux/BSD, these services, being desktop oriented on a multipurpose system, must be wholly decoupled from the rest of the system introducing a ton of opportunities for latency, stutter, and other issues to find their way into the user experience. It’s not impossible to minimize these issues, but it’s much more difficult.

Haiku doesn’t have to worry about this problem at all. It can simply implement the most effective and good-experience-conducive end to end design possible for its window management and compositing since it’ll never ever be a headless server.

Kinda I suppose, except without many of the limits inherent to those platforms. Both probably still aren’t as good as they could be for their purposes due to baggage inherited from their *NIX predecessors.