User blog comment:Braveangel/About planetary defenses/@comment-737801-20100429150753

Well the reason the Spectres exist was to keep an eye on the krogan, and they just expanded after the krogan rebellions. As to the Ascension needing help, any ship can be overwhelmed and need help especially with Sovereign on the attack as well. Battleships throughout history relied on support ships and the Ascension would be no exception.

To sum that up, the Ascension was vunerable to the overwhelming firepower that the geth fleet brought and any ship is vunerable to that.