There is no issue with splitting EXP between Party members, it reduces grinding in the endgame and is mutually beneficial for everyone.
...Did you spot the key part of that sentence boys and girls?
In the Endgame.
In the main game, it often results in you having a full team of 6 pokemon that are 5 or 6 levels over the opponents at all times. Back in X and Y when they had the system for the first time, I had a team of over Level 40s by the time I reached Korrina, the third gym.
For reference, Korrina's strongest pokemon was a level 32 Hawlucha. The Mega Lucario you get after the fight to introduce megas is the same level, It's suppose to be strong and powerful, and yet this one was 8 levels lower than the rest of my team.
The first Gym Leader that would actually challenge me level wise would be Valerie at the 6th gym, except she won't because I would be in the 50s at that point. This is before you take into stuff like the Amie Bonuses with the dodge chance, crit chance, shake of status chance, survive hit chance (Like anything was even going to kill me at that point).
Effectively, act 1 of the game was over and nothing was going to challenge me for the rest of the game.
So I turned that off for Sun and Moon, and with the old EXP curve, I actually stayed around the same level of the opponents and actually got to enjoy a small bit of challenge with the major fights (Helps that the Totems actually have interesting movesets to go against).
Then here comes Sw/Sh and you can't switch it off anymore. So now to avoid another X/Y Situation, I have to self impose a whole bunch of additional rules so I feel like the AI can have a fair chance. Swap pokemon between gyms, don't use any raid pokemon with their perfect IVs or raid rewards like the ludicrous EXP boost candies or the TRs with insanely strong mid and end-game moves before you even beat the first gym. No Camp Bonuses. No Dynamax in Gym Leader Battles (Seriously, if you Dynamax Turn one in Gym Battles you can often just Steam Roll it there and then when you get those stat and weather/terrain buffs)
And despite that, I still found myself outpacing the game level wise. A wild pokemon I caught in the route before the Fairy Gym (An Indeedee) was put in the team, did not battle once, and was already a higher level than the Gym Leaders strongest pokemon when I got to them. Even with all these self imposed rules, the game was still too easy with how much EXP it gives.
So should EXP be shared, I don't see why it shouldn't. But the system really needs to be changed so that it doesn't result in an overpowered team. Because if I get an overpowered team. I'm essentially just playing a Clicker game at this point. Just leading with my 10 levels stronger pokemon, hitting whatever overpowered TR I got in Slot 1. Not even needing to care about type advantages of weaknesses because they don't have the raw stats (or in many cases, an actual decent attacking move) to put up any sort of challenge and then, oh I guess I'm champion I guess.