Brief foray into the strategic game F&E: Initially the Feds get CA-based fast cruisers, but as time goes on, they have better use for the few CA hulls they can build (carrier, X-ship conversion) and more NCA-based fast cruisers tend to be built. If they build any at all.
Though there is something to be said about a group of Fast ships acting as a reserve; they can move 7 hexes to take place in a battle vs 6 hexes for regular warships. That and said Fast ships can keep up with a fleet that has X-ships in it, which also move 7 hexes.
This is not particularly relevant to ACTA:SF unless you are playing some sort of campaign game, naturally.