Both are too exploitable in large team games. If you could give ships, one player could spam ships and another spend on lots of upgrades. Then the ship spammer gives the other guy his ships and you have a fully upgraded fleet in half the time.
Where's the exploit? I don't see one, but assuming this is an exploit like you say, what would have been wrong with making the option toggleable? Or making it so that traded ships come with the upgrades from the original player? Why couldn't this have been made to work?
Even worse with planets. An economy player could just develop planets while in no upkeep, then give away planets to the military spammers once they start running out of steam. They weren't dumb enough to didn't make 'send resources' require a research in Diplomacy only to make feeding 100 times worse.
LOL. Why would the economic player want to give away planets to the mil player? You want the economic player to max out his eco, and to do that he needs planets. If anything it seems the reverse would take place - mil spammers would conquer planets and then give them to their feeders.
Your concerns are not well-founded, because in muiltiplayer this can already take place. Somebody can take a planet, then tell you to send your colony frig over, then he abandons planet and you take it. But this doesn't happen as part of some diplomacy or negotiation, and it is impossible to do with the AI in single player. The ability to do this as part of diplomacy would have streamlined the process, allowed SP folks to do it with the AI, and allowed for some kind of bargaining to take place (i.e. you attack this player or pay me this much money or give me this particular planet or that particular ship, and I give you this planet).
I just don't see the problem you are concerned about. Other games have had advanced diplomacy systems which included the giving away of units, planets (or cities), etc. It is something that similates real life (allies give each other weapons and what not all the time). The options could have been made toggleable for MP. This would have made the SP game better.
