the up/downvote system would be problematic in every other forum than this one due to the forums quickly becoming a popularity contest. however, in this forum it would be an easy way to voice support for a suggestion.

The problem with this is that it's not particularly useful for a suggestion. One of the things I always say is that this is the Suggestions forums, a place for discussion. It's not a ballot box.

Indeed, merely upvoting a topic would not contribute anything to the discussion. It is similar to posting “support” (and nothing else) on a topic, which is also discouraged as it does not contribute to the discussion.

The Scratch Team does not use peoples' support to determine what should be placed on the website. If someone makes a suggestion of “Add 3D Scratch” and manages to get 100,000 people to upvote it, the Scratch Team still won't add it, because it's rejected. Similarly, an earlier suggestion of mine, “Add Cloud Lists to the Rejected Suggestions Sticky”, would probably get a lot of downvotes if it were revealed to the general public. Yet, it was implemented, and probably would have been no matter how many downvotes I got.

Moreover, there aren't “front-paged” forum posts or anything. Therefore, upvotes don't really serve much of a purpose here.

I think it could also lead to a lot of advertising on the Scratch website of “please upvote my post”, especially if that user thinks there is some sort of meaning behind upvotes. It's not easy to do that now, because users will have to manually type in actual words, but with an upvote system, they can just click a button and move on. Hence you may also get “false support” for a topic if that user doesn't really know what they're doing.

Lastly, who's to say that there wouldn't be a popularity contest in the Suggestions forums as a result of this?

You are encouraged to not only say support or no support, as it says here:
One question, would you only be able to click it once, or would you be able to just spam the button.
Edit: looks like I wasn’t fast enough.

Tallented-Code-bot wrote:

You are encouraged to not only say support or no support, as it says here:
One question, would you only be able to click it once, or would you be able to just spam the button.

only once of course.