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)