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.