This is absolutely not the case, you can turn the cancelling on and off while still wearing them and it makes a huge difference to background noise. Many years ago they had a demo where you stand in the store and they blast a low-pitched white noise roar (like a jet engine) at you, and have you turn the cancelling on and off -- it's something ridiculous like a 45db drop. I bought a pair on the spot.
It's certainly the case that the cancelling is less effective at higher frequencies like voices than it is at low-pitch background noise, which does decrease their effectiveness in an office environment.
I have mixed opinions about build quality -- the originals were solid, I wore these around town most days for a few years, and they are now about 6 years old and still working. The replacement cups that I bought were garbage and disintegrated within 6 months of only at-desk wearing. So perhaps construction is not what it used to be.
Interestingly I heard that Bose patented the physically optimal geometry for noise-cancelling microphones, which would mean that their competitors are necessarily inferior at pure noise cancelling; don't know if that's true though.
I also find it annoying that these headphones require batteries to function even if you aren't using the noise cancelling feature; makes it very unreliable for carrying around town, but not a problem at my office desk. Without knowing anything about the electronics involved, it seems like it would be much better to have it degrade to being non-noise-cancelling powered headphones, instead of bricks, when the battery dies.