I included him because he's a good, near NHL ready RD prospect which along with Hamilton, Pesce and Kovacevic shows the Devils have a lot of depth at RD, which opens up trading Nemec. The reason I prefer to keep him over Nemec is because even though I think they're of similar quality Nemec will have much more trade value due to his draft status and unlike Nemec, Casey seems to have no problem developing in Utica for another season which makes keeping him and Kovacevic an option.
It's not about getting worse, It's about his development curve stalling and not improving at areas that were problems even in his draft season. If he was showcasing better defensive position, physicality or skating then I'd be more optimistic about his potential but with those warts still clearly in his game it makes you reevaluate his potential.
Nemec isn't good defensively either. They both have warts there currently but one player is on an upward development trajectory in that area and the other has largely stalled out in making defensive improvements. And if you view them equally or close to equal as prospects then trading the one with more value on the market that doesn't want to play in Utica next season makes more sense in both trade value return and roster flexibility.