Not any more uncomfortable than single tapping any other part of the screen one-handed. Of course this depends on screen size...
You're right, this would probably be perfectly fine on the iPhone, though it could be annoying on something like a Galaxy S3, especially for someone with small hands.
If it's inside that centimeter, zooming into the middle is useless anyway.
True, but if it always zooms to the middle, the user knows that they need to center the point that they want to zoom into before they actually zoom.
You're right, this would probably be perfectly fine on the iPhone, though it could be annoying on something like a Galaxy S3, especially for someone with small hands.
If it's inside that centimeter, zooming into the middle is useless anyway.
True, but if it always zooms to the middle, the user knows that they need to center the point that they want to zoom into before they actually zoom.