We don't recommend blacklisting any IPs for the reasons you state and others. Akismet is extremely accurate.
Do I have to maintain a blacklist?
Nope! Part of the idea of Akismet is that you're always protected up-to-the-second from the latest dirty tricks of spammers. There's no maintenance, no upgrading, no hassle. http://akismet.com/support/#ua1
The vast and overwhelming majority of all so-called information on the internet is spam. Over 80% of all so-called comments submitted to .wordpress.com blogs every day are spam. The worst of all spam containing malware and viruses is not sent to us. It's removed as it could bring down many blogs if opened.
If Akismet has sequestered these spam comments simply click "empty spam" button.
If you get spam that slips by Akismet then mark it as spam (do not delete it) and over time Akismet will learn it's spam.
On the spam that Akismet catches it takes only seconds to click "empty spam" and you don't even have to click "empty spam" if you don't want to. Akismet will maintain comments it has caught for 15 days from the time they were received, and then it will delete the comment automatically.
For more information please take the time to read "5 things every blogger should know about spam" on this page Akismet: How it works http://akismet.com/how/