I'm afraid I can't remember why I began writing "&c" rather than "etc", although I know I now prefer it (rather than it merely being a matter of habit) since it neatly separates the "et" and the "cetera" and looks cute to boot. Do you remember where you picked up the practice?
Dots are used to denote that the previous character is an initial for a longer word. Since "et" is a whole word you only put the dot after the "c" that stands for "cetera" so "e.t.c" is kind of a hypercorrection like octopi, ignorami, or habañero.
