Completely agreed. This is a serious issue and directly punishes players for working together. Two human players in the same area will make a lot better exp per hour by staying separated with 3 trusts each, than to team up. They would benefit from not partying even if they could summon trust in a party (which you said you'd let us do, but it wasn't added in this most recent update).
in fear of being called out for suggesting the game to be made "too easy" (kind of too late to worry about at this point in the game's life though), I really think it is time to make exp not be divided among party members, and just give all party members the same exp as they would have gotten solo. This should at least be the case within a single party, and outside of abyssea. That being said, I wouldn't exactly mind if it applied for an entire alliance either. I guess it makes sense to not include abyssea, because those areas use completely different EXP mechanics anyway. Also, exp is more than good enough in there already.
While we're at it, we should be allowed to use Fellow NPCs in level synced parties.
Historically, it wasn't an exp penalty. EXP over time would almost always increase when you partied, solo exp was usually completely rotten because you had a lot of downtime and took forever to kill monsters alone. The problem is that the exp model hasn't changed along with how the game has changed. Right now, it makes sense to not divide exp among party members, but back in the day, the current model already gave you around 4 times more exp in a party, and using the model we're asking for now would have caused oldschool parties to gain 24 times more exp in a party compared to solo.