Hi, Here's my opinion. Directory submission serves very little purpose for getting direct traffic to ones site. However, it may help in link building a bit (in some cases). That way it partially helps in ranking. Social bookmarking on the other hand is more useful to link building as well as getting some direct traffic.
I think you can get quality backlinks and more stable search engine rankings or pr with directories submissions. Comments can also build backlinks but with my little experience with comments my pr can go down easily with just this method. I don't know about other person but this happen to me. I think traffics and link building can be separated to 2 different things. In terms of link building with achor text you may submit to directories or social bookmarking but in terms of traffics you can go for comments.
directories are good tools in getting PR, social bookmarking are good ways to rank in search engines. both are important in seo.