Quiver of Marksman's Aid - useful?

Description:

[...] shoot 40 units of ammunition. Though this ammunition does not deal any damage, it marks the target on a hit, thus all other companions get additional 2 bonus on each attack against this enemy. This effect doesn't affect the initiator even if she equips other quivers or weapons. Only one creature can be marked per encounter.

I just got this and have some questions:

• Is it 40 per day, or just 40? I think just 40 :(
• by "does not deal any damage" does it really set the archer's damage to 0?
• If just 0 damage, it seems kind of useless to give up a full round of archer damage for a mark. Does this have any real uses?