Real world results
Read Anand's article on the subject. They determined that even with the 'flaw', reception on the iPHone 4 surpasses the iPhone 3GS or other phones. THAT is what really matters, not the number of bars.
That coincides with my experience. If I go out of my way to do something I wouldn't normally do, I can make the number of bars drop. If I use the phone normally, I get a LOT fewer dropped calls than I got with the iPhone 3 (actually, I haven't had a single dropped call with the iPhone 4 yet) and call quality is better.
Look at the geometry. If you want to hold the phone with the lower left corner cupped in your palm while making a phone call, you have to twist your arm so that your elbow is in the middle of your chest - not the most comfortable way to make a call. If I simply hold it normally for making a call, my fingers are no where near the black stripe.
Giving away bumpers is not a cost issue. It's simply a matter of Apple not wanting to do something stupid. Bumpers won't solve the problem and most people don't have a problem, anyway. So why should they tell 2 M customers that there's a problem and here's a free bumper to fix the problem when only a tiny fraction actually experience the problem in real life? That would be terrible PR.
It's a great phone. If you don't like it, feel free to return it for a refund.