You SHOULD use the speaker function when the phone isn't held to your mouth.
I KNOW I SHOULD, as it's designed. That's the problem. I SHOULDN'T.
For 10 years voice microphones have been progressing, and the current sensitivity when the phone is not in a strict position is a setback.
Considering Apple decided I MUST choose between holding the phone in a STRICT position or go to speaker mode, NO ALTERNATIVES, then yes, I SHOULD keep my mouth and ear stuck to the phone, even soaking wet my ear when outdoors.
But forcing users to change the way they speak to be able to rely on the sound quality is not the way to promote a new feature. Both should be compatible.
It's not just my opinion. They are facts:
1) Getting the voice through both mics makes the voice canceling unusable
and
2) The proximity sensor automatically switching the screen on-off means that Apple ALREADY determined that YOU SHOULD be able to move the phone away your ear, and without having to "TELL IT" when,
THEN,
IT'S A FAILURE of design not to use the same sensor to PREVENT you having to tell the phone you want the noise canceling stop FAILING.
The hardware is ok. It's the despotism by Apple designers that want to tell you how to use their phone, or else
Well, no, actually it's not the designers but the marketers who are looking the other way because the noise canceling is the big marketing point now.
If I was so idiot by not switching to speaker mode just to put the phone away my face while talking and it was SO OBVIOUS as you say
Why do you think Apple STILL wants me to choose it by hand?
They could easily switch it on-off with the proximity sensor! and help me to be less dumb.
or why doesn't just Apple make me switch the screen on-off manually?
or why don't they just turn BOTH the speaker and noise-canceling feature on-off automatically with the proximity sensor?
Thinking consistently is what makes designs evenly good. This doesn't make sense.
For the ones who tried it out, are probably just ignoring the lower volume, which compared with the iPhone4 (don't know the 4S) sucks.
If you make the test with an iPhone4 in front of you, you'll notice it has a greater voice volume for the same position.