I have no doubt that if the interim tag is taken off of Gibby in the offseason both Howell and Stott are gone. The downside to that, and I want them gone, is that will be three different hitting and pitching coaches the players have to deal with in three years.
These players have had intense coaching in competitive high school, college and minor league baseball. The impact of coaches at the ML level is minimal, IMO. Every now and then you get a guy like Charlie Lau and Leo Mazzone who can have a huge impact on team performance, but they both required hitters/pitchers make serious commitments to their philosophy (and often times major mechanical changes).
The real benefit of the guys on the bench are the sharp-thinking bench coaches who really know the game, usually pitchers and catchers who've spent their careers having to think about every aspect of the game at a pitch-by-pitch level. Pitching coaches? Hitting coaches? These guys are cheerleaders and A/V specialists. I assure the success or lack thereof has almost nothing to do with who fills these positions for the D-Backs.