Why blogging platforms should introduce DNSBL at network layer

I truly wish that blogging platforms would introduce DNSBL lists. DNSBL are usually referred to as blocklists, blacklists, or blackhole lists. Basically, they’re lists of known spammer IP blocks, and they basically deny access if there is any access coming from those addresses.
There’s a good reason for this from a technical perspective. If you actually host multiple blogs, then one of the problems that becomes an issue on your server resources is that with each comment/trackback spam that is logged, that’s a little bit of your resource taken up. If you calculate out the additions of each insignificant portion, all of them added up become a lot of wasted cpu cycles that could be used for more useful things.
Noted, if you block with a .htaccess or what not, then you would still be committing some resources to processing ip checks. But this would be far more efficient than actually running the complete spam through your blogging platform, then marking it as spam. That makes it inefficient.
That’s the inherent issue with blocking spam at the plugin level. And currently with the TypePad Anti-Spam and Akismet, I have to say that it does drive some serious cpu cycles that I could be using for other blogging efforts.