Shhh, you're going to start the "merit sources are people too" movement. Nobody wants that.
In all seriousness, I don't think its too much of a issue. I try to just merit those that are posting quality content, but at the end of the day it does feel nice meriting someone who might actually need them, and its even better seeing a newer user that you've merited in the past, actually get motivated by it, and turn out to be a top quality user.
If I was in the predicament which I'd nearly exhausted my conjured merit source, and it was between a merit source with a very good quality post or a newer user with a slightly less quality post, but good quality I'd probably give it to the newer user, because it might encourage them to continue. I can always go back to the merit sources post at a later date, which I typically do revisit a lot of old threads, and replies to merit. I've also seen this behaviour from other sources too.
I wish our local board also had a merit source. Though recently that thread is getting spammy because of a bunch of new users but still many quality posts have been unmerited there because higher-rank local member usually doesn't show up there regularly. Maybe this is also a reason why our local members ignore the local board. As you have mentioned everybody wants some kind of reward which will motivate them to go back to that place again.