As a reader I like the idea of having a comment feed per post, but as a blogger, and developer of my own CMS, I can’t get used to the idea that each reader would load a dozen (or thousands if they’re greedy) of feeds every hour (or ten minutes).
I’d rather not imagine the load a Wordpress site has to endure if you activate that functionality :)
In fact, I’d rather blogs published one comment feed, with a specific tag attaching each comment to its specific blog post, and let RSS aggregators do the math. Makes more sense to me.
My point being, you know that some aggregators will be poorly programmed, and will fetch your feeds every ten minutes and ignore the cache or http headers; and the more opportunities to abuse your site you give them, the more they’ll take. Whereas making one and only comment feed is not rocket science — and it degrades nicely: it’s pretty usable even if the aggregator doesn’t explicitly support it.
I’m unfortunately a little burned in regard to the particular category of bloggers who’re liable to weigh on the evolution of standards, so we’ll have to wait until someone more respectable has the same thought.