A:  The simplest reason is that it is squarely at odds with Yale's Information Technology Appropriate Use Policy which specifically prohibits this behavior.

More satisfying however, might be the answer that since your Netid and password are used across so many different applications on campus and in so many ways that effect your privacy and security that it would be very inappropriate to share your Netid password with anyone else. There are almost invariably better more secure ways of collaborating and sharing with others.

For more complete details and some practical tips for choosing a good password please see:

http://www.yale.edu/its/security/Articles/Omnibus/netidsec.html