I think you've stumbled onto the point right there. Banks have been promoting more associates directly because of competition from PE/HF firms. People with BB experience and a top MBA degree are ripe candidates for PE/HF jobs. I think the reality is that it's still only the top 5 or maybe 10% that get promoted directly without an MBA.
The bottom line is that you'll have your chances to land an IB job from any of the top schools. It's a very traditional job path, and if you express your interest and commit yourself to the process, firms are looking for people like you. If your talking about private equity or hedge fund jobs, Wharton and Columbia (and of course Harvard and Stanford) will have advantages over most of the competition. If you're looking at investment banking, the pool of candidates at Wharton and Columbia (and certainly Harvard) will be hyper-qualified. Yes, firms love people from these schools, but you'll be swimming with the sharks when competing with your fellow students. Not an easy task unless you have the right background.
I disagree with the notion that the "top 5 or maybe 10%" that get promoted directly. The buy side will always lure away the best performers. I don't think you can really generalize on the direct promotes except to say that they are definitely not the poor performers. I think it's much more a function of the individual analyst's career goals and aspiration as that of being career bankers.
I agree with your bottom line regarding having an opportunity to interview with BB IB from any top school.
I disagree with the swimming with sharks hurting your chances theory. If your end goal is to get on as many closed lists as possible, and not to actually get the job, then I might agree. But once you are in the interview process it's about the individual and you are competing with all other applicants regardless of which school they come from. Additionally, I think that at the hyper-qualified schools IB is not the first choice of the ultra-competitive finance oriented candidates anymore, it's now PE and HF.