Blog commenting is a lot different from what it used to be...mostly because blogging is a lot different than it used to be.
A blog can now basically be a relatively complex online publication...but it can also be just a small, "traditional" blog. Both are useful to the online marketer, depending on how reputable and relevant they are within a given niche.
In terms of marketing, blog commenting does have its returns, but they might not be as directly tied to ROI as some marketers would like. When it comes to building authority and making connections across the web, though, blog commenting can be a really good thing.
As brands go about building awareness and authority online, one of the best ways to do this is by joining the online conversation. That's why posting in forums and commenting on blogs can be a really helpful tool for marketers wishing to improve their clients' brand visibility and authority.
It's also good for networking!