It was explained in the blog a year ago, and I guess things have not really changed:

User-Agent           Requests      Bytes Served
Yahoo! Slurp/3.0       56,331     1,124,861,780
Googlebot/2.1          56,579       773,418,834
Mediapartners-Google   30,519       671,904,609

...and then, after some specific settings did not help:

In the meantime, since Yahoo (via Slurp!) is about 0.3% of our traffic, but insists on rudely consuming a huge chunk of our prime-time bandwidth, they’re getting IP banned and blocked. I’m a bit more sympathetic to Google, since they deliver almost 90% of our traffic, but it sure would be nice if they’d allow me to at least schedule the massive web spider storms for off-peak hours...

I wish I could I find the article I read about 3 months ago. However, the short version is that Yahoo is using Bing's index (entirely) and algorithm (mostly). Yahoo is still crawling with their own bot to make small amendments to the search results and for other projects.

In short, banning the bot could drop you entirely out of the Yahoo SERPs (but would have no effect on Bing).

UPDATE: This thread on webmasterworld has some interesting posts.