No one is saying we shouldn't group with new players or that new players aren't a welcome growth of the game. What many are saying is that they are a huge burden to a veterans progress and maybe they should give more incentives to accept the mission with them to compensate for the liability, more than a flat 20%. I often decline missions that state I am getting a mentor bonus, mostly because I'm burnt out on the previous missions where low level players single handedly costed the mission. I can only accept losses so many times before I get tired of it.
EDIT: Of course from a developer's point of view they could just say, "Well we could just as easily not have ever implemented a mentor system." And that's true, but one uses the system now more as a warning not to accept the match than an incentive to accept. The issue of low level players being a liability (due to many different issues) would still exist regardless of the mentor bonus.
I can't help feel bad for the new players when I decline the mission, because on their side it says that another player did not accept mission (and that'd be me), and wonder how often this happens to them and what MSE could do to alleviate my inclination to decline a Mentor "warning". Especially since BP are more rare now, card shuffling and forge are more expensive now, and faction points are smaller than they've ever been. One may not have the luxury to "afford" to mentor new players. I don't envy being a developer and balancing issues like this, I can completely see how this would be a stressful job to handle.