First, where did you find that paragraph? That is far short of an article if you ask me. Forum signature links do provide some value SEO wise, not much but it does exist. I'm sure the more relevant the forum to website the more value the link my contain; in theory of course. Unless the forum itself is using nofollow attribute in signatures there isn't much to worry about, even so they can still provide a decent source of traffic on relevant forums.
Signature links are useful if you ask me. However, I think it takes quite a few (posts) to make any real major improvement in SERPs.
I see nothing in that article that can't be said about most other sites/pages... If you know what you are doing Sig links can have an impact on your SEO in more then one way. Posting in stickies for instance can get you some juice, as the forum gains PR, then next step is the stickies to gain some. Once that is done, your link can only be better off. Posting first, either as the OP, or on the top of additional pages. The closer your link is to the top, the more it will count. Relevance, anchor text, and most other SEO points still apply. If a sig link does not amount to much based on what that article says, then frankly links within posts won't count for much either. Which makes foruming rather a moot point Granted, a forum sig won't count as much as other links, but completely discounting them because they are worth less is just silly... yeah, they may count for less, but by the gods I have over three thousand of them! LOL