Well I beg to differ. By releasing two products at once it would force a greater number of people to hesitate before buying the MBA. As for whether the two products would compete, I also think they will. As a casual observer of this website I've already seen a few people asking "Should I buy a MBP or a MBA?". This is evidence of competition for part of the market. Of course, I suppose Apple won't really care as long as you buy one of their products, but I think it's not exactly true both products won't compete for at least some of the same market.
Finally, you don't think people purchasing the MBP want to show off at all? Although I suppose that question is immaterial in general, I found your statement a little much to be able to resist passing comment on.
I'm not saying they couldn't release both at the same time, I'm just saying it wouldn't make a whole lot of strategic sense (particularly from a marketing standpoint) to release them both close to one another.