"For example, the mere fact of supporting user accounts will mean having to design sign up, sign in, and “I forgot my password” screens as well as all the different error states associated with them."
I'd much rather have UI guidelines, which cover how forms are built, what an error state looks like, and how text is formatted. A set of building blocks, rather then a unique design for each activity being undertaken.
In my experience designers who design each page individually usually provide designs which can't be adapted easily, and require each page to have much more work applied then would otherwise be necessary.
By all means provide a full design for key processes, for example the checkout process on an e-commerce site, but for a lost password form I should I be able to put that together without having to call in a designer.