It's true that the bonus isn't much, but I suspect that most relatively new PM members see "bonus for expending all votes" and think "Kewl! Bonus points!" without really doing a quantitative analysis to determine the actual return on investment (of effort). The problem, after all, isn't as much the XP gained (though, as I mentioned, the gained XP does contribute to another "problem") as it is the gratuitous manner in which votes are cast.
Having a limited number of votes to expend causes one to consider whether a given node really deserves the vote you're going to give it. Getting rewarded for voting replaces that caution, for many people, with a desire to find excuses to vote on things. Getting bonus rewards for expending all votes compounds the problem by causing people to consciously think "Ooh, I need to go find excuses for voting until all my votes are cast, just in case I forget to vote before midnight!"