This is dangerous, of course, because it is natural that a connection between the two windows will be created. This way they will be percieved as two parts of a single site. In fact, they have nothing in common - except the phish site's copying of the color/font scheme of the legitimate one.
Also, this approach allows the phishers to easily hide the address bar in a phish screen - which is generally the most vulnerable spot of the scam.
However, under a closer inspection, the truth comes out: |