Modesty is a virtue.
As I said somewhere on page 5, I really cannot argue that.
I also don't deny that some dedicated concentration is required to spot behaviour patterns, which would point towards untruthfulness, for example.
However, I do believe, that this concentration is done pretty much automatically (as in you simply cannot shut it off completely), especially if you are trained in that field of expertise. So, even if the concentration is active, I would still see it as a passive skill roll.
And note, that Sense Motive still has active uses as well, as you pointed out earlier.
Something else...
Re: social skills versus PCs
Of course, I am also of the opinion, that NPC social skills should not influence PC behaviour.
For example, an Intimidate check might maybe result in some penalties as explained under Intimidate, but surely wouldn't make the PC act in any specific way. Likewise, Diplomacy cannot just change the PC's attitude to friendly. That's the player's right to decide and I wouldn't deny them this.
However, unless the player already has some knowledge about what is being presented to him or her, he or she cannot decide whether an NPC is being truthful or not. That's where the Sense Motive skill is used instead of the player's judgement. Because without a skill roll (or the DM telling the player instead of a skill roll), it's impossible for the player to derive any information of this kind.
So, when an NPC tries to bluff the PC, a Sense Motive roll is normally used to decide, whether the PC spots anything in the NPC's behaviour, that alerts him or her. Of course, modifiers, up to +20 to the bluff check, could make it very hard to do so, for example if a complete stranger states something, which is quite believable.
Looking at it from another perspective. You have stated somewhere above, that you would dislike a crappy diplomacy roll to stand in your way, if you performed a fabulous speech as a player. I agree with that, and I have no problems to use the roleplaying instead of dice rolling in that case (altho the opposite is true as well, if a player just can't express him- or herself that good). Ok. Now let's get back to Sense Motive.
Say, you have spotted something that alerted you in the DM's roleplaying of the NPC and you request a SM roll to see, if your PC notices something. Isn't that similar? I mean, you already have that feeling, that something is wrong, why roll for it?
Bye
Thanee