The more that a team can interact and iterate, the better you're final product can be. In my ideal world, I'd skip a detailed wireframing exercise and get to building a prototype as soon as possible. Its so much easier to explain what is difficult to build if there's something to click on. Or you might learn that what you thought was hard turns out to be easy, or vice-versa. Finally, if you already know if you are using Sharepoint, Drupal, or anything else, you're front-end designer has to know assumptions the platform makes, how it manages content, and other implementation details.
In my practical experience, I find that teams are more efficient when roles overlap and people understand what is happening outside of their silo.
