I mean, let's say you want people to go RP with newbies to help get them into the game. But people often tend to cluster, to RP with their trusted friends and existing RP partners, yes? And if just pointing out, "Hey, new players probably would like RP too" hasn't worked, you presumably need some sort of additional factor to help change that behavior.
Do you punish them if they don't? "Hey, I see that you haven't played with a newcomer to the game in at least six weeks. You know what that means; it's time for a public shaming!" I don't see that working to help people welcome new players; I see that being a great way to turn your players into someone else's new players.
Do you reward them if they do? It seems more likely to succeed, but... what form does that reward take? A nice post on the bboards? "So, Susan RP'd with three new players this week. Everyone applaud Susan!"
Or is a more tangible reward more motivating, one that they can redeem for something later? Maybe it's XP, maybe it's karma/luck points that can be spent to buy your way out of a bad roll, maybe it's a sort of 'staff time' currency that can be redeemed for "Hey, I want to spend 30 staff points to have this specific thing GM'd." (Though I personally kind of hate that last one. I feel like if there's a currency that can be used to buy staff time/attention, it should probably be something available to all players, not something you need to jump through hoops to earn.)
It's great to say "we're here to RP, so we don't need incentives to RP" but... demonstrably, we are often creatures of inertia. It's easy to play with the same old people, or even just sit idle and chat with the same old people. Want to change any of that behavior? It'll probably take either a deterrent (for those who don't engage in the behavior), or an incentive (for those who do), and of those two I would vastly prefer to be handing out incentives than arbitrary punishments.