It's been pretty obvious the last several years that the criteria some fans use to evaluate whether a player should be in the lineup is different than what the coaches seem to use. In particular, I think that's true about bottom-6 spots. This isn't limited to the Rangers between AV and Quinn. It's all over the league.
The criteria is just different, and simple on-ice performance isn't the only factor. When you get to the 4th line, it might not even be the most important factor. While we usually think of intangibles as being things like leadership, I do think a lot of what decides 3rd and 4th line ice time are intangibles: work ethic, energy, intention, personality, coachability... all of these things weigh just as heavily for lower line spots as performance. Factor in that Howden is still developing, combined with the things we've heard about him, having that leash actually makes a ton of sense.