He has seen soft competition nearly throughout his entire career and has seen very favorable offensive zone starts over the last two. Even with that his possession numbers have been subpar. He is the 14th worst forward over the last 4 years at driving play. In addition, he has been a drag on every single linemate he has shared significant amounts of ice time with over the last 2 seasons.
Even before the addition of Stewart (and aside from Westgarth), the Kings had a load of bottom 6 talent to choose from. Dwight King, Jordan Nolan and Kyle Clifford are all big forwards that would play a similar role as Stewart.
Nolan and King both proved with their play last year that they deserve roster spots. They in fact boosted Mike Richards' possession numbers after their late season call ups, and were pivotal players in the post-season. Kyle Clifford has been inconsistent over his young career and hasn't provided the scoring touch that Stewart has shown in the past. Yet it is hard to picture the Kings sitting Clifford in favor of Stewart, especially since Clifford's underlying possession work is better.
That being said, it is possible that Stewart could hang around until Kopitar returns from injury. The prospect of that is a little frightening, given that Stewart's poor underlying numbers would suggest that he is undeserving of that much time on the Kings roster. In the short term, the team would be better served giving a prospect a shot to earn some time with the big club rather than potentially allowing Stewart to weigh down the bottom 6.