[dogscoff]

By the time you have the capacity, and the opportunity, to train ships in such a way, is it still worthwhile to be using CSM barges?

Well that's true. However, seekers remain viable throughout the game in many mods, and they do also have their uses in the standard game: For example a few CSMs thrown into a fleet can help clog up the enemy's point defence if you are using a fighter-heavy strategy.