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So, if you are ranked well with Google, you could do alright for a while without being in Yahoo!. But, being in Yahoo will help your PageRank in Google and hence your findability. Therefore, I would want to continue in Yahoo.
Also, who knows what will happen in 2 months, 6 months or longer. I wouldn't want to risk things changing back to Inktomi or some hybrid mix of results. Therefore, same thing, would want to stay in Yahoo! for the time being until it could be proven that it isn't necessary.
Somehow, I doubt that Yahoo! is going to let their business be destroyed by people deciding they don't need to be in the Directory anymore. They are smart folks.
Well, that's my 2 cents anyway.