Developing a website takes twice as long if it needs to look and feel the same in Internet Explorer as it does in any actually standards compliant browser.
Mozilla isn't perfect either, it still has quite a few web standards issues. You could make your web site look nice in IE and Mozilla and it would still be broken in Amaya.
I don't have anything against embedded gecko except that it's a bit heavy. Any blitzmax app that uses it will balloon in size.