One of the responses to the question about the mass of football spam said:
The proper course of action is to flag the post as spam. Three spam flags will remove it from the front page, six will delete it. Don't edit it, don't downvote it, don't use another flag. Flag as spam and move on.
Assuming I've already flagged an obvious spam post, why shouldn't I downvote it as well? Is this discouraged because enough downvotes will hide it from the main page and potentially make it more difficult for users to spot (even though search engines will still index it and penalize the site's rankings?)
Are there other reasons? Is the rationale different for lower-traffic SE sites?