I haven't invested any SP on just a couple of skills.
Horror
Corpse Plage
Some people might use this skills, but I don't have them and I don't regret it.
Some might say that you can actually use Corpse Plage on pvp when you cat gets killed or while doing aoe. Or using horror on pve while agroing more than enough...but I prefer hitting UD and facing the opponent.
Horror hardly lands (46 DA on white/green mobs), very few mobs run away. Mage fear is more reliable - not 100% just more. Maybe if I equip DA with some mage setup it would help though, haven't tried that. Male tank in FP running with Ghoul's staff may be 1337 :p
I was trying to think of a funny reply to this but couldn't so....
the truth is that DA skills are reasonably solid and most of them can be used in some way or another. More to the point for DA--I haven't had ANY problem having the SP for skills at each level.
At lvl 58 I had more than enough SP to upgrade to every skill I cared to get and get each level of those.....and I don't die too often so it should be just a function of the skill costs and SP you roll up as a tank baby.
The obvious skills to avoid at low levels are power shot (bow) and mortal blow (dagger) if you're never going to use those weapons.
Personally I'v always felt hamstring was useless since it never lands.
You don't have to sweat DA skills--at 58 and 60% i have all skills available to lvl 58 except last hamstring (115k sp) and the last 'horror', and the mortal blow ones...doesn't seem to be much of a problem for our class.
Horror is a very usefull skill when you solo. At level 68 I have used it many times. Granted it does not always land but I can say it lands in a 1:2 ratio and when it does it saves my healer's mp.
Maybe I have a better landing rate because I have GreaterEmpower but regardless I wouldn't minimize this skill. Maybe when Interlude comes out it will have a good pvp landing rate.
But honestly, I'm on my 3rd version of my DA subclass, and on all three occasions, Skill such as hamstring/Dark Panther you can skip until you plan on getting actively involved in PvP.
If you find yourself with buffers, or in parties a lot, of have access to your own SE/Warcryer at least, you can skip Corpse drain skill.
Skills I always get are - My Weapon and Heavy Armor masteries as well as other Passives, My Drain, Shield Stun, Shackle, Final Fortress, The Self buffs available - after that, pretty much optional. I myself do learn Corpse Plague as well, but the XP I usually do its useful for.