In a short game on a small map RA/DA is not worth researching at all. However on huge maps, and in long protracted battles then DA comes in handy. In games like that your fleet caps will more than likely be maxed out anyway. Along with all of your researches. You probably will have tons of credits, and resources to spare. Especially if you control a star system.
Obviously if you built up enough ships to where you have little to no fleet cap left then there is no point in using DA. Just divide your fleet up and phase gate them where needed. However in a big battle it can be useful to re-enforce your fleets if they are taking heavy losses, and you need to replace those ships right now. Instead of waiting for your factorys to build them. Which by the time the newly built ships arrive it may be too late. They can be used stop gap until your main fleets can arrive, but you are correct in that the forces are not enough to hold untill your fleets get there. Unless you use DA on multiple gates on adjacent planets.
With the advent strike craft increases in 1.1, I dont think a few extra sentinals would hurt. Granted i do agree they shouldnt spawn in DA. Sentinals are easy to replace without needing DA. I have seen mostly cruisers with a few rare instances of an all sentinal spawn.
I agree that the cost needs to come down a some. I do like the suggestion that the summoned ships should leave after x amount of time, and return to phase space. The units shouldnt cost any fleet cap at all being that they are NOT part of your fleets. The same thing bothers me about the iluminators deceptive illusion. The fake ships use fleet cap. This would make for some interesting fights if someone forgets about the dark fleet ships in your group, and they dissapear in the middle of a fight because their time expired 
Returning Armada is not totally useless. It just needs some more work, and tweaking.