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I think the primary reason is that those who have transitioned to the new system will not have this information (as it's not available moving forward), and those who haven't transitioned would have... creating a disincentive to transition and an advantage for those who don't transition...
i.e. If you're in the new interface and bid $x per click but can't see competitive bids and I can... i could bid $x - $0.01 and 'jam' you without your knowledge
Wonderful timing how Yahoo changes how advertisers have to bid right after they change their search engine algorithm and throw people who had enjoyed natural rankings several pages back.