Crock pot use
You should keep the lid on as much as possible. You are correct; once you open the lid, the liquid inside can quickly lose temperature at a rate of 10-15 degrees per minute.
Additionally, taking the top off allows steam to escape, which reduces the point of cooking in a slow cooker (cooking in a relatively closed system, with the steam condensing back to water and flowing back into the broth).
It's not doing a whole lot to the flavor (aside from causing the stew to thicken with the loss of water), but it is definitely causing the cooking to take longer than it should.