The answer here is that it depends on the profession. Leatherworking specialties, for instance, are 100% completely useless. Engineering specializations, by contrast, have a significant impact on the cost of levelling from 425 to 435, provide access to some of the coolest and most sought after vanity items in the game, and in Cataclysm, determine which of 2 new BoE pets you can make and often sell for absurd profits. Alchemy specializations meanwhile, result in an average of 20% more yield from all specialized recipes - hugely relevant, and really the only way to turn Alchemy into a substantial moneymaker.
As for Blacksmithing specifically? There are no new patterns linked to specialization in Cataclysm. Unless you're interested in some of the old vanilla or BC era BS items for vanity purposes, you can safely ignore specialization. I seem to recall a few mid 70's blue items linked to specs as well, introduced in Wrath, but they are not efficient skillups.
Tl;DR: They're irrelevant for all except Alchemy and Engineering, where they provide significant moneymaking opportunities.