This is similar to Should we allow downvotes on Meta feature-requests if they just don't *like* the *feature*?, but that question was (incorrectly) closed as a duplicate of Encouraging people to explain downvotes and Down-voting on questions - Require a comment below a certain rep?. Additionally, I feel this question could be more clearly asked (or I misunderstood it).
What is the significance of an upvote or downvote on a feature-request?
Like the rest of SE, the title text for upvote is
This question shows research effort; it is useful and clear
For a downvote,
This question does not show any research effort; it is unclear or not useful
But for feature requests, some believe that an upvote means
I agree with this feature suggestion
and that a downvote means
I disagree with this feature suggestion
These seem quite different to me. When I see a feature request that is well researched, clear, and perhaps even useful (in that it is valid, and a reasonably good idea), but with which I ultimately firmly disagree, should I vote it down?