At my company, as developers, we rotate taking support tickets and working directly with customers on the issues our (very capable) support team can’t handle. We and our customers are both very happy with the results.

What you're describing is customer success, which is almost always post-sales. Engineers working with customers post-sales is a great idea. *Pre*-sales is where it gets tricky.

Yeah, this drives value for almost all roles. No need for a separate focus group when you can ask the people who are already using and/or paying for your product.