Normally what a frontline does is provide your team with the damage you need to accomplish your goals. In a sinsplit, you don't have a frontline because your goal is just to stall until VoD while your sins clean up NPCs. When VoD comes and the NPCs march on you, the NPCs provide the damage that the sinsplit lacks nicely making up for that weakness. Prior to VoD, they have enough shutdown (in the form of eles or illusion mesmers) to lockdown any frontline you have. They don't do damage, but neither do you and that's to their advantage. They came in not wanting to do damage, so mission accomplished for them.