I don't think there's a reason to fear someone else will go back in time and try to give their side an advantage, because, for one, blowing up a star with Red Matter and traveling back through the wormhole isn't exactly the safest method of Time travel, plus, it would result in the Traveler being sent to another alternate branch.
However, there is a risk of making of things worse. For instance, let's forget Prime Spock for minute, who could share more advanced tech, but, let's say someone else traveled back and prevented the encounter with the Borg at Wolf 359. If the Federation hadn't been devastated by the Borg, they wouldn't have ramped up their military tech, and therefore, would've been conquered by the Dominion, because Wolf 359 put them on a footing where they were able put up a good fight. Or maybe stopping the Doomsday Device 3 years earlier, prevents it from destroying a Civilization that rises up to eventually destroy the Federation. Even saving someone, could result in the saved ending up leading to a threat down the road.