I mention this as a public service.........when you watch a game, watch the line play. There is a reason that Mark had 1/1 stats. The d line couldn't keep the o line off of the linebackers. I am not making an argument for or against clark. Although I think heap would have exploited him. I think the reason the coaches aren't on mark the way some fans are, would because they understand that the d line is not holding up consistently.
Also something worth mentioning.....putting clark on the field would not change one position, it would change seven. If Clark isn't playing it could very well be because he can't make the defensive calls, doesn't know the d well enough. The MLB position entails so much more than tackling the runner. You may just have to trust the coach on this. If Clark doesn't get people in the right position, the d will be giving up a hell of a lot more yardage vs the run.