In older games, I hated grinding for EXP and battles just to only level 2 Pokemon at a time. It's one of the reasons why I have a bad memory of playing Black and White. It's also why I used to love overleveling my starter, a practice a lot of kids do and one I took into my teenage years.
With the upgraded EXP Share to level your whole team at once, it makes it so much easier and more rewarding to decide on a full team and level up 6 Pokemon at once. It allows me to level up something other than my starter, while also keeping my starter around the same level.
I for one love the change that they made to the EXP Share, but I wish they had a toggle to turn it off if people wish. It's probably the most common sentiment I've seen for the EXP Share. Changing how it works is fine, just as long as you can revert back to the old style.