A combination blade is the one I don't have one of Herb. To me it's a case of compromising in exchange for versatility. It's not as good at ripping than a rip blade or crosscutting as a crosscut blade. I know that there is some economics involved for many people but my opinion is that if you can manage it, you will be better off in the long run if you buy one of each and get blades that are the best for what you want to do. The buying "one size fits all" is a case of short term gain but long term pain to my thinking.
Stick covered the other question better than I could have.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.