According to Why shouldn't I downvote spam that I've already flagged?, spam shouldn't be downvoted.
But what about a completely opposite action, that is, upvoting it. This has the advantage of spam being more visible and more likely to be quickly removed.
No. You're giving spammers reputation, which they can use to do nasty things like posting comments with spam links (which tend to go unnoticed on old posts), post more spam links, posting spam on meta where it's less likely to be seen, upvote each other's spam posts, etc...
This could be a potentially dangerous action.
Upvoting gives spammers reputation, as Glorfindel said, which could allow spammers to be more destructive. But he missed an important point that I'd like to add here.
You know it's just a matter of a flag when you see a screen full of incoherent text advertisement, but what about other things? Remember, spam isn't the only problem Stack Exchange is facing, but also rude or abusive content. Those do more harm than spam, especially to legitimate and regular users of SE.
On some sites, new users cannot post images (I guess SO is one). If a spammer has any rep, they could post spam images, which are more disruptive. Those images could even be NSFW stuff like pornography or violent content that makes people feel uneasy.
Bad images just caused some issues on Stack Overflow recently. See this MSO post: NSFW triage review audit (Fell free to click the link if you have 10k on SO - the image is already deleted)
There's literally no reason to do this.
Let's consider the worst case scenario. You're the only one who realises you can flag. You flag. A dozen other people downvote it into oblivion.
It gets kicked into the flag queue. A mod considers it, thoughtfully considers it, and nukes it - and often the user who posted it.
Practically? There's sufficient incentive to flag these things and to make them go away. And of course there's Smokey.
A single flag, on a healthy site with at least one active moderator should be enough to get it looked at and handled. Upvoting for "more visibility" is pointless when a single flag will bring it to the attention of someone who can dropkick it into /dev/null.
