It is true that Dirges are invited to groups to buff and debuff, but that's not our only job. We're also there to DPS and can do pretty decently. There are a lot of places that putting points into our personal DPS outweighs improving group buffs. I personally use this but I know a lot of Dirges don't like my bard tree. Most Dirges will use something more like this.
Even in heroic zones, it comes down to little numbers here or there that change how much we help the group. Just trust where points are, either of these specs work, though my personal one is set up for grouping with a Warden who has a better blocker so mine was rarely needed. My Bard tree would look more like the second if I didn't have the Warden with me 75-90% of the time.