Yeah it depends on his strength. If he is just some edges stronger then keeping the AI at war will keep them also weak all the time. But if a single AI has so strong ships that he won't even loose shipfights then it's only a matter of time until he's going to successfully invade planets and by that becoming even more stronger.
Actually, speaking of this, I remember an other game (I think it was the Drengin Gambit) where the Altarians had a Precursor Library on their homeworld, and became immensly powerful. I was able to conquer the Torians very early on, so there was only the Arceans & Altarian left - both stronger than me. Although I had 40% of the planets in game the suicidal AI outsearched me fairly, the Drengin in TA have a bad economy + research abilities, so in the long run, this game would have been lost.
But I brought the Altarians to war against the Arceans - and built some very fast Transport Troops and positioned them along the Arcean border (to not provoke a yet good relation...). The Altarians send some fleets and took away all Defenders & fleets, leaving the Arceans entirely defenseless. I then send in my Transports before the Altarians got the chance to - conquering the Arceans in a single turn.
After that, I had 75% of all planets - which, in the long run enabled me to win, but I did never ever use that strat again.