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.

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...

"Hammer" is subjective, but there does seem to be a general belief that Slurp is let's say...maybe a little more aggressive than it really needs to be, and that it's been going on for years, ie. 2007, 2009, 2011. A basic search for something like slurp robots delay will turn up a giant pile of complaints.

In case you're tempted to actually block Slurp, as suggested at one of those links, you might want to also see this recent question here for a different take.