Some cars have worse blind spots than others. I don't buy those cars generally unless they are really really good cars. The Lotus Elise, for example, has some of the most useless mirrors and suffers from huge blind spots. The rear view mirror on the Exige is completely useless. And yes, convertibles tend to be even worse, I've owned one and those blind spot mirrors were useful on that car but on most sedans I've never felt them to be necessary.
My car has excellent field of view and with the mirrors adjusted properly I can easily see a vehicle come up from behind, follow it into the side view mirrors and then into my peripheral vision. It also has a backup camera.