Former SEO consultant here. This is a fine, low-effort low-reward way to build links. The reason it works is it is not actually that easy to write the kinds of comments that will make it on a worthwhile blog in the targeted industry. The person who does this is doing better than average unless the goal is a difficult one. The attention and traffic from comments actually part of conversations is real.

Even if the keywords targeted are very difficult, this is a fine activity for idle moments or for a marketing specialist/writer to do.

SEO is one of those types of skills you really don't want to be outsourcing to SEO firms. And why would they be the ones creating the link worthy content in the first place?

Well before I built my product for my current startup, I was an SEO consultant. And this is a very good, albeit simple, technique I've used. That's why I shared it.

Most people fail at SEO because they simply aren't persistent. This method breaks down the process to the point where it's the easiest it's going to get to stay consistent with your link building efforts.

Also, it's really easy to avoid bad links with this method: If a site doesn't look legit, just skip it.