I think the sole criterion should be the benefit the post brings to the forum and its members. As you can see, I have awarded merits to two legendaries in this thread.

You are very generous and I do see your point. However, I see merits as a valuable and scarce commodity and if all else is equal (which it usually isn't), I'd give my smerits to lower-ranked members over Legendaries. Hero members still need them, but Legendaries can't rank up any more than they have. The problem has always been finding those low-ranked members who make decent posts. Often I find myself doing what you and Vod do, which is giving out merits without regard to rank or need. It so happens that I find posts made by older members a lot more interesting.
Foxpup, for some reason, has given me quite a bit of merit and though I don't "need" it, it is always nice to have some smerit to give out.
Anyway, since smerits are very limited--especially if you're in the lower ranks--it's understandabe that you'd want to be selective about who you give it out to. I think that's perfectly acceptable.