I rather suspect that the reason for "oppose" votes comes from the voting formula:
http://meta.wikimedia.org/wiki/Wikimedia_Foundation_elections_2015/Vote_Ques tions#How_are_the_winners_determined.2C_and_what_is_the_process_once_voting_ is_completed.3F
which declares the winners to be those with the highest
Support/(Support+Oppose)
As they say, "do the maths" (or if you are American, "do the math").
If you are just voting to support anyone who you feel would do a reasonable job, you have no real motivation to cast "oppose" votes. But if you are particularly seeking the election of a particular candidate, then you increase their chances by casting oppose votes for the others.
Just as a simple example. If there are 2 candidates, Andy and Betty. Let's suppose 10 people think Andy would be a good choice and give support votes for Andy. Let's suppose 10 other people think Betty is a good choice and give support votes for Betty.
Andy's score = 10 * (10+0) = 1
Betty's score = 10 * (10+0) = 1
Both are equal.
If just one of Andy's supporters gives an oppose vote to Betty, what's the situation?
Andy's score = 10*(10+0)= 1
Betty's score = 10*(10+1)= 0.909090
Winner: Andy.
It's strategic to "oppose" other candidates. It's not necessarily saying anything against the opposed candidate personally.
Kerry