To what end? No, really. Spam already gets reported and dealt with, so what do you propose we downvote? Posts encouraging bad practice? Without downvotes, upvotes become meaningless.
Posts encouraging bad practice or that are poorly thought out or misleading, certainly. Spam may already be reported and dealt with but there's also a category of post that falls somewhere between spam and the useless that isn't really dealt with currently.
@weirdan "karma-like" suggests the aim is to give people whose posts are in receipt of upvotes a warm fuzzy feeling, whereas I see it as a way of making the forums distraction free. For instance, some relatively straightforward UI changes mean downvoted posts could be greyed out or otherwise reduced in prominence (but still be available for those who wish to read them).
That said, if this has been discussed lots of times in the past then so be it