I delete so many every day at various forums.
2 post wonders!
and those 2 posts are not even related to a thread they are dumped on.
Oh, delete and ban.
Adding a sig to YOUR OWN REAL PROFILE is a perfectly legit method of giving a site a little traffic boost (note the word "little") if your posts garner attention and leads people to follow the link. I check out links of members in forums I frequent when their posts suggest they know their stuff and the site in the sig looks interesting.That's what profile links are for. They're a way to let other members find out more about the poster. They're a privilege the forum owner grants to members in return for participating. Nothing even mildly black hat about that.
That said, nobody will ever click on the link of a 2 post wonder or someone that speed posts fluff for count. And frankly I have serious doubts there's a gain in having a link for a Florida real estate company or a spa in Redondo Beach under posts that originate from the opposite side of the globe. All that tells me is some idiot is too lazy to post and too stupid to spend his marketing budget wisely.
Building stacks of dormant or useless profiles in one forum is frowned on if not prohibited by forums worth visiting, so a "link building campaign" that employs intentionally breaching and willful attempts at deception is by definition "black hat".
Another danger of this practice is the temptation presented by access to multiple profiles. Its just so easy to use them to bump threads, ask leading questions that the other id can use to showcase the product, or use them to praise the service of your other id. Nothing will destroy your credibility quite as fast as having someone point out that the guy that just complimented your integrity is really you. It is used to boost credibility, but when detected, demolishes it.
Like others that have spoken I have serious doubts about there being any boost for a site available from a massive profile spam campaign. In fact, it just tells people the owner of that site is not trustworthy, and if one persistently re-adds new spam profiles with the same link it paints a bullseye on that site for the mod team that raises red flags when that site is mentioned, so it accomplishes the exact opposite of a good marketing campaign.
As for using power tools... If you're looking for an seo and he proudly points to his Xrumer profile campaign, It's just a sign the "professional seo" involved is in reality not even a talented amateur. Just smile and put him on the list of vendors you will never work with, cause he's a wannabe.
Bottom line, profile link building is great if your goal is to take aim at your own foot and pull the trigger. It wont fool google, and it wont fool readers, so why do it? It has no effect if done in moderation, a harmful effect if implemented vigorously, and if the mods find out you're the one doing it you won't even keep your own real profile. Waste-o-time.