(1) for an "at will" employee (that is, most people), there is no obligation to provide two weeks' notice, just as there is no obligation on the part of the employer to provide severance pay (unless it is stipulated in the employee handbook).
(2) Most people leaving a job want to have a good reference from that position later in life. You never know when you will need it. Also, it is not uncommon to be looking for work in the future, and that former employer could well be the best available option. You don't want to burn bridges unnecessarily.
(3) Any new employer who demands that you "start immediately" should be looked at very carefully. They are in most cases causing hardship for the person's existing employer and in the absence of a real emergency (exceedingly rare, given that they position is vacant anyway), the company is probably fckud up in more ways than one.
(4) it is simply inconsiderate to leave with no notice; it is a hardship for the company, co-workers, clients, and so on. A mature, responsible person would only do this under extreme circumstances.