Our main objective to occasionally bookmark isn't to try to rise above the crowd with a popular "digg" or what have you, but to gain more valuable backlinks. Not all of them register in the search engines, but a few here and there most certainly can help give you a boost (again speaking of only backlinks).
There are just so many social communities out there (not unlike this one) to engage in that bookmarking hoping someone will "discover you" might be more work than it's worth. It's time consuming even with automated tools (which may or may not bring value), and time is valuable
