Character-bound karma may have been implemented as an indicator of an individual character’s advancement. But, with all the other currencies being account-bound, it serves to no such purpose, as I can buy anything else for any of my alt and I can disregard karma for most of my characters.
But, making karma-bought items account-bound would lead to small portions of unusable karma on different characters.
Therefore, I think it would be much better to have a karma pool from which all characters can benefit, just like all the other currencies.