It kind of seems like the least interesting possible thing to do with Hurd, given its major architectural differences from Linux, is to make it as much like Linux as possible. I'm just not sure who the target users are for "Debian GNU/Hurd" or why they'd ever be better served by it than by Debian Linux?
Right now, the target audience are Hurd developers, who need a useful userspace to work with. Making that userspace as much like Linux as possible reduces the incremental maintenance effort, which is critically important given the limited number of developers.
(You could reasonably ask the value of developing Hurd at all, but given a set of people wanting to develop it, it makes sense for them to do so using a Debian userspace.)
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Right now, the target audience are Hurd developers, who need a useful userspace to work with. Making that userspace as much like Linux as possible reduces the incremental maintenance effort, which is critically important given the limited number of developers.
(You could reasonably ask the value of developing Hurd at all, but given a set of people wanting to develop it, it makes sense for them to do so using a Debian userspace.)