Sounds like a pre-optimization. You can get the same level of information with two comments and vote exclusively with +1's, but without the general negative aspects of what's mentioned above.
Right, if you're asking every single issue poster ever to remember to post two comments specifically for voting, versus just posting their issue itself.
Not every issue needs an up/down vote. But with the addition of -1, all issues and _all comments_ on those feature requests now have one. Which takes us back to the negative consequences outlined in this thread.
