Back when I was 16 (right after I got my drivers license), I was driving on a two lane road, mostly staying in the blind spot of the person next to me (oops). After about a quarter mile, the driver decided to merge lanes without looking over his shoulder (he probably just used his mirrors, but since I was in his blind spot, he couldn't see me using the mirrors). He nearly pushed me into on-coming traffic, but after our cars hit, he reacted quickly enough to get back into his lane before killing me.
Lesson: don't trust your mirrors. There might be an inexperienced and/or poor driver in your blind spot. And don't think that you can keep track of the cars behind you. Cars can come out of nowhere.
If you do decide to only use mirrors, at least use one of these (or similar):
Amazon.com: Total View 360 - Adjustable Blind Spot Mirror - As Seen On TV!: Sports Outdoors