To me one of the most important things about a cellphone is how good the reception is.
With a 900/1800/1900 cellphone, you're only getting reception from the 1900 MHz towers. Rogers (the towers SO uses) has probably covered Canada assuming that their clients are using phones with both 850 and 1900 MHz.
So, you can use a phone with only 1900MHz (I think most of the older Fido phones were like that) but your reception may suffer.