It's not all black or white. It all depends on whose shoes the IM doc is filling in taking that shift. If he is covering a shift where they would otherwise have no physician, it's a positive. There are many undeserved EDs, with few or no BC/BE EPs. An internist should not feel bad about taking such a shift. If the internist is taking a shift where there's only an extender to work, no physician, again, there is value there. Though I've worked with some really good EM extenders, it's best to have a physician on duty, or if you prefer, physician and extender working together.
Now, if the IM physician is attempting to compete with, or is displacing a BC/BE EP, ideally you want to EP, who is residency trained and also has much more training in peds emergencies, and acuity.
That being said, I'm sure there are some internists that do a great job in the ED, especially with the adult stuff, and some BC/BE EPs that need improvement, but we have board certification for a reason. Get the piece of paper. It means something.