I'm not sure what the hours are for the checkpoint at the DTW Westin, but the couple times I overnighted at DTW and needed to use the Westin checkpoint, it was closed .. at least in the morning.

As far as allowing hotel guests into the terminal, I suspect it's a moot point to some extent because if you're staying there, you're probably using the airport anyhow - but being able to access the facilities in the concourse is a plus.