Considering that ships can theoretically emerge from a subspace portal in any direction, it makes sense to have something like the Iceni guarding over it - the frigate's firepower would be more effective in dealing with larger threats at a range, while it can provide supporting fire to friendly fighters and bombers.
The problem is that the GTVA is probably still the dominant force in every star system they control post-Capella, both the Terrans and Vasudans have cruisers and corvettes that might be able to do the job just fine (e.g. Naxos in Love the Treason; Thebes in Bearbaiting; Nebtuu in Exodus), and it's probably more cost-effective to send fighters in to do scouting first, before sending in bigger guns (The Sicilian Defense). A frigate might fill the gap left behind by all the warships the Alliance lost in Capella, but then there's the cost of manufacturing an entirely new line of frigates based on one example that no longer exists. It seems like a huge risk compared to the continuous manufacture of ships that have a proven combat record.
