Point the First: I think you're overlooking the fact that there's more than one person of lower ranks here. Granted, the lower levels still don't add up to as many votes as the higher levels, but they aren't insignificant in this respect.

Point the Second: It has been suggested that it gets one into the habit of spending all votes, which I think carries a fair bit of truth. Eliminating the habit-making incentive will surely help cut down on later over-voting as well.

Point the Third: The first couple of voting levels pretty much never have any members to speak of. I think the earliest couple of levels pass too quickly. Ditching this extra XP incentive should help slow that down slightly. Granted, I'm an odd case, having posted a couple of things early on that got a lot of voting attention, but I literally advanced three ranks in two days. A little of that involved the fact that I got bonuses for expending all my votes.