I think it's a myth fostered by a few on here that guest stars are hampering character development or costing characters storylines/jobs.
Vincent (as the newest character) is being developed really nicely. Paul, whatever you think of his budding relationship with Ben, is developing nicely and both of these characters are involved in big storylines as well.
I think the problem with the 'well rounded new characters' is that people don't have the patience to wait for them to be developed...they want immediecy. Of course when a character comes in and is developed quickly like the Carters, they tend not to be happy with that either, it really is a no win situ, I think.