The advent rely (almost exclusively) on abilities and how they stack to win fleet to fleet battles. Repulse is designed to separate large fleets from the groups of support, very useful since the advent are almost always outnumbered by the vasari and TEC.
Oddly, I don't find Advent to be that outnumbered in multiplayer games. The only time I see this happen is when someone gets cap happy. When I play Advent I tend to make a lot of ships.
The problem arises when a single cap is targeted and cannot move at all. A level 10 cap can be brought down by 5 guardians and 10 disciples (a little bit of an exaggeration).
Just a couple guardians messing with a caps ability to turn and move in the face of a fleet is the problem.
Repulse is a double-edged sword though, enemy ships are no longer it the range of ruthlessness, and the synergies are innefective. It also spread out the enemy ships, meaning that the whole advent fleet has to turn to focus fire.
Repulse is most valuable against HC...you wouldn't necessarily use it against long range frigs, you just roll over them instead. People do use it to try and split up caps from their support.
The entire advent fleet is a tad overpowered by the late game, and the entire vasari fleet is a little lackluster.
Yeah, Vasari are a little lackluster now that they lost the Subverter edge....if it wasn't for phase missiles, they couldn't stand against Advent at all. But TEC are really dependent on their HC's late game to stay competitive, and these are totally countered by skillful use of Repulse. The longer a game goes, the more able Advent is at countering just about anything it faces. This is why so many Vasari players now just build an Orkulus everywhere -- a strategy I hate btw.