At least for me--someone unsatisfied with the bloatware del.icio.us has become, willing to try something new, and really not too fussed about a signup fee lower than MeFi's--the cost was actually an attraction. It says "We're serious" and "We want to keep out spammers", both of which are fairly crucial to a bookmarking service.

It also makes the site stand out: probably only URI shorteners are more common than social bookmarking services, and the increasing signup fee differentiates it from all the others (I doubt I'd have given it a second thought otherwise).