To say "I'm mostly concerned with only 90% of my visitors" seems quite reasonable to me. We all have impossible deadlines, and we can't test every configuration as thoroughly as we'd like to. So where are you going to put most of your effort? Making sure 10,000 have the best experience, or looking after 90,000 of your visitors? Where are most "potential customers that just might want to buy something" more likely to be, in the 10,000 group or the 90,000 group?
I suppose it's all about priorities. If you ignore IE because you know there are better browsers that everyone should be using, you probably won't get many jobs. It's like people creating websites for Macs because they might think everyone should make the switch. IIIBradIII, you're 100% right about learning the specs, but browsers do have their quirks. Stick to the specs, you'll have a good base to work from, but concentrate 90% of your development effort on what 90% of your users are going to see. Or, be mostly concerned with only 90% of your visitors.
It's good to keep an eye on trends, but be realistic too. Look at what people outside the web development community are using, especially non-techie folk. Chances are, that's your audience. Chances are, they're not spoofing user-agents. Chances are, they've never even heard of Mozilla. Chances are, they don't even think in terms of browsers, but in terms of 'getting on the internet'. Easiest way on most default configurations is clicking that Internet Explorer icon...