In DA there are 3 ways i will structure a fleet.... if the AI is using powerful fleets then i will use tiny hulled ships backed by military starbases. If i am going to take losses one way or the other, then at least they should be cheap losses! I will remain on a defensive stance untill my technology pulls its finger out. Also along this same line i may also choose to throw in some more powerful ships to gain experience while the tiny hulls do most of the fighting and all of the dying..... once again, if i have to take losses, they may as well be cheap losses!!
Now if i can get a powerful ship with enough defence to withstand enemy firepower, i may choose to fill the fleet with the same or, if i don't have enough ships yet, i may as well fill the logistics with tiny hulled ships that have reduced firepower so they are not targeted first.