Why? Most phones other than the iPhone don't support true multi-touch and it hasn't been an issue for them. The only major use case where I've seen it be a problem is with games that have controls that need to be pushed at the same time.
Double tapping to zoom works fine... in fact I can't tell you the last time I pinched on my iPhone, I always double tap.
Typing on mono-touch displays has not been historically a problem because until now you had to type with one finger or a stylus. If you want to type with both hands and more than one finger, mono-touch will be an ocean of pain.
Double tapping to zoom works fine... in fact I can't tell you the last time I pinched on my iPhone, I always double tap.