Since it said once per round, you could use an opportunity attack or an attack granted by the Warlord, before your turn to deal curse / hunter's quarry / sneak attack damage. This change prevents that, *if* you got to apply that extra damage during your previous turn. It's slightly different book keeping. It also lowers the damage output potential of the strikers a smidge, because they will get slightly fewer chances to apply sneak attack damage. The damage potential now resets at the beginning of their turn, instead of at the beginning of the round.
Actually their damage should go up slightly at the beginning of a fight and down slightly at the end of a fight.
1/round means that if a friendly granted you an attack before your turn you could get sneak attack on it. But you could not get sneak attack on your turn.
So at the end of combat when the monsters are dropping there is more of a chance that a friendly will grant you an attack before your turn comes up which can then sneak attack. Such you could then get that extra sneak.
But at the beginning of a fight you can actually get this attack with sneak attack damage because you have not used it since the end of your last turn.
E.G. Init order
Warlord
Rogue
Wizard
Actions:
Warlord grants ranged basic attack: Rogue Sneak attacks
Rogue attacks with CA: Sneak attack rests at beginning of turn: Rogue Sneak Attacks
Wizard does his shtick.
While at the end of a fight you could have had under the previous rules
Warlord grants attack: Rogue sneak attacks
Rogue attacks with CA: no sneak
wizard does his shtick
new round:
Warlord grants attack: Rogue sneak attacks
monster dies, rogue does not take a turn.
End result should be slightly more damage as you are more likely to be able to manufacture attacks before your turn starts in combat(iirc) than you will be able to manufacture situations where the monsters die ahead of the rogues init.