It's mostly a number game.
Even if your CTR (Click-Through Rate) for those buttons is very low, you'll get several clicks if you receive thousands of visits. So they are generally worth having.
Just be careful, as I caution in the book, too many buttons dilute your call to action message and lead to a lower CTR.
Sharing/social buttons are nowadays used for submitting stories. And if resubmitting old stories by visitors is not happening often (+), then it's pretty stupid. Why? If I'm already submitting my posts myself to social sites, these buttons should point to already created there entries. Even by only assuming that votes are the main concern in such sites, it's easy to tell that having "splitted" entries instead of the main single one is purely wrong. But crucial benefit here is social consolidation, so your visitors can easily check reddit or HN comments, if there are any.
And I believe my idea is really good, yet I don't understand why people go with mostly useless submitting buttons requiring additional work of the precious visitors. I also don't believe I could be the first one with such simple yet efficient solution.
(+) It's kind of the question here, I don't know whether there are any statistics about it.
Generally speaking, intelligent plugins/buttons already do that for you. If the article hasn't been submitted already, they let you submit. If the article was already submitted, they let you upvote.
What you think about self-promoting vs sharing overlapping issue when same channels are used? Putting buttons allowing reader to link your text on reddit, HN, etc. seems redundant and may even lead to duplicated entries there, which is bad. So should these buttons link to entries author already submitted herself/himself? (Twitter is out of question, as it has a bit different distribution model.)
Advantages of direct linking to particular reddit/HN/etc. entries are pretty obvious and I am somehow amazed not seeing this in the wild. Are there any serious downsides of such solution? Question could be rephrased to: how often old stuff is rediscovered (i.e. linked for the second or more time) on reddit/HN? If it's extremely rare case, then common (at least nowadays) sharing buttons are rather useless.