I guess it's a bug, since it's very silly: I was helping a heavy patrol or security fleet from Tri-Tachyon (with a Paragon, a Doom, some light carrier and a few frigates) chasing a pirate armada. They engage first, then I join them. The Tri-tachyon fleet choose to deploy everything except the Doom and the Paragon! Why on earth it should do that? Even after trying to not deploy too many ship on my side to get enough point so it can deploy one of these big ship, IA refuse to send any new ships until all the others one were destroy.
It's not the first time I see this problem. Each time I help an ally fleet, it will first deploy its small and useless ships instead of any usefull cruiser or capital.