Ok... let`s look it at like this. Yes Google is better and will far out due any search engine. It is worldwide accepted like irene says. But my point is that yes you should do a lot to make sure all your stuff shows up on Google, but at the same time shouldn`t you be doing at least a little on other search engines? I mean that is just more free traffic you can get. Check out your stats... In one of my sites Yahoo is my number one traffic bringer. While another it is Digg. And another has StumbleUpon. The rest are google. But in all the ones that have google as number one. I am still getting great traffic from other directories.
Like yesterdays stats on my blog about credit managment
I had 334 people come from Google
210 from HubPages
199 from Yahoo
123 from Digg
And then they start going down under 100... but look at it. Now granted I payed to be in Yahoo`s, but that is still 199 people that I didn`t get from Google. And if you are selling something, you know it is all about getting as many people in front of your product as possible.
That`s just my .02