Even though DS is lacking in damage for PVE, she has a lot of opportunity to dps, more so than 'almost' any other class. With an hp regen plate for sadism pleasure, the heal cap is removed. This means that 9% of all damage you deal heals your character. If you deal 10 million, your character will heal 900k.
This also means being able to 'tank' a lot of mechs that require players to stop attacking in order to survive. Some examples are:
DDN Manti tornado - If you have the SA skill jade for helms, you can dps inside the nado.
DDN Scorp spinning attack - Just spam skills and your SA won't be broken.
TKN 3rd boss multi-bombing + map-wide aoe - If you time your skills correctly, your SA won't be broken, and you can simply blitz/rage over the map-wide AoE without having to run to the safe green line, then follow up with a chain claw to immediately ground yourself and continue comboing.
IDN Kodiak poison + confuse - You need to spam like crazy an maybe throw in a blitz/chain claw combo if you're lagging a bit.
PKN final boss poison cloud - Just keep attacking in the cloud - no need to run.
DDL final boss Hide&Seek - Again, just spam your skills - don't worry about getting bubbles.
Even though it's not 'game changing', the extra opportunities that DS has to attack are useful. Since you won't be dodging at all, you can save your dummy ghost for when team mates are in trouble and you need to take aggro off of them. This can, in some situations, also give the rest of the party opportunity to attack. Having the boss on you most of the time isn't bad at all as a DS.
Post Awakening, DS will be able to keep the Phantom Avenger buff up 100% of the time, provided you have a CD plate for it. This means that all other skills will have their CD's reduced by 30% for the duration of PA (which is 100% of time while not stuck in a mech like IDN's 2nd boss's spinning laser + green bubble safe.) It will also be possible to have the damage boost from grudge up near 100% of the time, as well as phantom guard. So no, DS can't really contribute buffs/debuffs to the team like her SE counterpart, but she does allow the entire team, including herself, to attack more often. DS might not be the 'ideal' class, but she is still somewhat 'useful'. Aside from AH, DS also levels the fastest due to her sadism pleasure buff and large AoE and linear AoE skills. If you wanted you could level as DS to save time, and then class change to something else if you don't like her.
Plus, being somewhat
immortal is actually quite fun, and would allow you to learn all mechs/attacks for the rest of your characters.