I've provided IA and UX direction for nearly 200 websites over the past 4 years. The dynamic between a UX and developer depends on both the designer and developer technical skillsets. Some IA/UX designers solely provide wireframes, flowcharts, and mockups. Great UX designers move their role forward by providing rapid prototyping with working knowledge of HTML5/CSS3/jQuery/JavaScript. This being said, I believe the ideal dynamic between a UX and Developer is to achieve the proposed goal, using the least amount of resources. Use whatever you have to as a team to get there, so long as you get there.