I guess I must be one of the “some people say” group. For indeed, buying “targeted traffic” can get you lots of exposure to the exact audience that will most likely create natural backlinks, which definitely helps your SEO. However, whether it helps you reach the top rank, or not, also depends upon how good your content is, how much SEO was performed on the content, and how relevant and useful it is for the targeted keyword.
The OP’s question was about cheap (untargeted and/or fake) traffic, which is a different story. Untargeted traffic isn’t likely to help your SEO and is just a waste of time and money.