Voting is a privilege not a right !
Downvoting as weapon to promote a different answer you upvote is, I suggest, a violation of the Be Nice principle on SE. I'd suggest a pattern of this behavior ought to be grounds for moderator action - a warning, then maybe more serious action if the individual persists. I do think this behavior is destructive enough to warrants a ban if the offense persists.
The argument that it's not explicitly against the rules is a false argument. It's a violation of the be nice principle and that is a fundamental rule on SE.
If an answer is reasonable, and even if you don't agree with it, the nice thing to do is vote up the answer you prefer (or none of them).
If an answer has a fundamental flaw (in your opinion) then it's legitimate to downvote, but downvoting in the "nuclear option".
Two deep flaws with the argument of those promoting downvoting-as-tactic, IMO, are :
This would turn the voting process into a popularity contest, rather than a vote for what is considered the best answer by consensus. We would be in danger of people downvoting against particular posters, rather than their post. It's a very slippy slope and all downhill.
It will turn people who post decent answers away because they are getting negative feedback despite posting a reasonable answer.
I'd suggest that adding a comment explaining the weakness you see in a post is the correct approach (or voting up a comment that already says that). It is extremely negative to just downvote and not explain to the poster what you think the problem is.
Note that "I downvoted because I prefer this other answer" is such a negative thing to do to another contributor and just because you prefer an answer to another does not make it flawed in a fundamental way.
While reaffirming that votes do not require motivation
Any downvote must have been motivated by something. If there's a logical reason to do with a fundamental flaw the voter sees in the question, then downvoting without explaining why achieves nothing in the long run. We must recognize also that a small minority of people will abuse any privilege they have for their own benefit (including ego).
So my argument boils down to : a pattern of behavior can indicate a motivation and that revealed motivation can be treated as a violation of the "be nice" principle if appropriate.
I'd prefer keep the people who try and post useful answers and respect others, than let them be driven off the site by people who think we're here to validate their egos and opinions and want to force their own opinions by gaming the voting system.
how do you feel about this usage of voting: upvote and downvote to widen the gap between the good answer(s) and the less good answer(s)?
A terrible idea that punishes good answers and hence punishes good posters. This is the last thing we should be doing on SE. It should be actively discouraged where possible.