Having SOMEthing is all I ever want. AO3 is enough, as long as it actually has content. It's hard to tell nuance on likes and preferences if you have no fic, no bookmarks, no retweets/reblogs, no content-producers that you follow, no fandom presence whatsoever. Like, everyone wants "good fic" but I don't know what your definition of good fic is if I have no examples to follow. So personally, no, I don't really find essay letters that helpful enough in that regard? But it's not like you're breaking the rules or anything, it just means you're more likely to get something closer to the writer's taste than yours.