That isn't very practial, because doing a-la-carte things like that doesn't take into account the fact that different paid account features cost us different amounts to offer, and are "priced" based on the idea that not everyone will use all of the features adn not everyone will use the features to their full extent. (The message i wrote about this a long time ago, to the pre-beta mailing lists, is no longer available, but you can still find a cached version of it on the Wayback Machine.)

But I can totally see, and make a case for, a "commenting lite" type account for people who don't want to maintain an actual journal here but still want to either a) support the site or b) get access to certain of the (commenting-related) paid features. I just don't know if it's a compelling case or not.