Crafting specialization allows SE to simply do 'more' with the crafts then they currently can. Maybe they could add bonuses to the specialization that they don't want to stack, under any circumstances? Or add pieces of BoP gear or glamours that require the specialization, so that no character could have them all? Or, as someone else mentioned, add some sort of 'request craft' to make specializations useful to each other between characters, not all in one? All of these would make mastering the crafts with different alts nearly useless, as you still wouldn't be able to do it all 'for' one character.

Even if they didn't do any of that, specialization would prevent complete mastery on one character, or trade or economy or whatever reason they see as important. Sure, people 'could' create 8 characters to each master one craft a piece, but the sheer amount of time, effort, and gil required to do that is 'massively' higher then doing it all on one character, especially up front. Whatever their reasons, specialization will certainly get them there.