The purpose of the useful arrows are from what I understand, to get the most useful and/or the best answer near the top while the not useful near the bottom.
It's a good system but I noticed that when people started getting useful points down voted(the down arrow) some people got offended that it's more of a personal level (like people giving the down arrow votes didn't like the person or answer)
That is not the case right? The point of the down useful arrow is not to punish someone on a personal level, the point is to get the best answer up, right?
Because in most situations, a negative(-) sign means bad. So instead of showing how many points are (-), just show 0 but hide the actual the amount of (-)arrows so people don't get the wrong impression or idea about this system. The system will still work since this will encourage those people why their answers are near the bottom without offending themselves.
Also I seen this type of system work in Starcraft 2 where the devs hide the loses so players don't concentrate so much on their loses and concentrate on getting better and winning.
Again this is from my observation and suggestion on this since I seen a few times when people getting down voted and not knowing why they got down voted and asking why or get a little offended over it.