I’m not sure if it is appropriate to call playing in a group a 300% more multiplier for loot.
As mentioned before, there’s class specific gear that drops that doesn’t make sense to gift to everybody.
Next, I personally will not stop on every item that drops that I can’t use and check out if any of my team members want to have it. I’ll set my loot filter and blow through the game. There will be lots of usable stuff for others left on the ground. A group of 4 will have better chances of finding specific items. But if they want to take full advantage of it they will also take x times longer to complete content because they constantly have to check items between each other.
You also seem to always look from from the perspective of somebody that gets feeded by all other party members. Do you think this will happen? So you play with somebody and he drops a good sword you could use on your Sentinel. But what if the guy that drops it also plays a Sentinel. Or he plans to do. Maybe the item is so damn good that the dude thinks “wow, I’ll just create a Sentinel next to use this sword”.
He doesn’t get anything in return from you unless you have something similar useful for him dropping for you so you could simulate a trade by gifting stuff to each other. But normally nobody would have an incentive to share his loot with you. It’s not that everybody on the internet and in games is nice and wants others to be happy., I guess.
And last, it’s not a real more multiplier for loot. You drop the same amount of loot as if you play solo. So does everybody else. Its instanced loot for the party, not shared. It only becomes shared if you do what I mentioned above and stopp after every mob group to post item links in chat and gift thinks left and right. This is not very realistic. People will only stopp if they see specific uniques drop. Maybe not even then, if you’re playing with strangers that just want to clear content fast.
The only thing that makes MP kind of “abusable” is when groups of friends commit to feed specific things to each other, like boss drops. So doing a mono boss run to acquire a specific item can be used to increase chances if the group commits to give the desired item to one guy in the group if it drops.
If you’re playing with random people, most likely they themselves will have a specific goal when doing boss runs. So if you’re farming a specific boss and join others that do as well, its likely these people are looking for the same items that you do.
So if you have friends that just play the game for feeding you, you will have an advantage.
Else I don’t see it as a problem. MP always has a benefit compared to solo play. This is the nature of MP. If you want to artificially erase any of these benefits, its not MP anymore.