If a blogger stops posting and effectively abandons their blog then comment spam that escapes akismet can build up without being dealt with. So even if one spam in 1000 gets through, if the blog is no longer monitored, the spam will be posted.
Sensible bloggers turn off comments on older posts, but fair weather bloggers can (and do) abandon blogs with no further thought.
Something that would help this would be to have a feature that after a fixed time period, the ability to make new comments, trackbacks and pings is stoppped.
I think this should be in the core rather than as a plugin, because it is precisely the bloggers who try blogging for a while and give up that would not bother to search out plugins or consider this.
A step further could be that as the 60 day limit is reached, some (maybe last 45 days) comments are deleted in case any of these are spam.
Also a warning could be posted to the admin of the blog to warn them of the situation.
The time limit could be customisable if bloggers had special annual posting schedules. 60 days is just a suggestion.