The danger of all monopolies. I'm not trying to troll or promote irrational fear, but I feel that the competition has been too weak.
The hegemony of a single organization which has a clearly stated philosophical viewpoint with a desire to index all information quite simply poses risks which others have described better than I can claim to. By diversifying those risks I hope we can reduce the potential problems.
I'm not saying that Microsoft is the antidote, just that I am glad to see competition. I hope there is more forthcoming.
Try getting customer support with Adsense or Adwords (almost all of Google's money source). It'll suck or be non-existant. Oh...that is unless your in the 1% of top spenders.
Are you worried about corporate shenanigans, or a single point of failure or something entirely else?