The site that I am working on has a signed-in and a signed-out state. We're in the process of implementing a soft signed-in state (e.g., remember me on this computer). Doing that requires users to re-authetication to access certain areas of the site (e.g., user profile). One of these areas has some terms and conditions that are presented in a modal. When the activity in this area intercepts the user and requires a sign in, makes the current page based sign in form hard to slot in for this one situation. One train of thought is to present the sign in form in a modal but this forces us to implement the modal sign in across site for consistency.
I'm not a fan of modal because I think modals are an easy way out so to speak. We don't rely on modals when designing for mobile devices... Also, so far the site has had a page based sign-in which is a fundamental change in the user experience. The goal is to make it easy for users by presenting the sign-in in the context of the activity but on the other hand users may find it more of an interruption than redirecting them to and from the sign in page.
We don't have the option of user testing... I'd like to get your feedback on the modal approach.