Agree that it should allow for better role playing. For myself, I invariably end up playing a "good guy" so diplomatic usu fits the bill, but I did strangely feel like I couldn't mix it up sometimes, even tho I technically was allowed to. The lack of dominant tone will be good for players like me, so I can just pick whatever response seems to most appropriate for the situation.
I kind of had the opposite problem. My Hawke used Sarcastic as her default with most NPCs, but she would often use Diplomatic with Fenris, Leandra, or Carver and sometimes Aggressive with Anders or Merrill. (She actually cared a great deal about Anders and Merrill both, but she was very worried about them so she would try to use tough love.) Because the game interpreted her as primarily snarky, though, she would occasionally take a tone with one of her "exception" NPCs that utterly didn't fit when I wasn't in control of her response.
My most disappointing dialog moment in DA2 was when I didn't get to select tone of the final response to the apology about the ruined night together from Fenris in Act 3, because the game chose wrong and that wasn't what she would have said at all... and it kind of killed the moment for me in what was otherwise probably my favorite scene.
With the non-chosen dialog being neutral in tone instead of trying to match dominant tone, I won't have to worry as much about that happening.