Barking Alien has posted an article regarding why its important actually split the party in a game from time to time. Most of the time when this advice is given it’s down to either the players putting themselves in the position where they are going to potentially suffer from the consequences of this action, or because its going to put extra administrative load on the GM, who is often already dealing with a hundred different things to help make the game flow. The blog post does draw out an important reason to allow it from time to time: to allow individual characters to shine outside of the group and to allow themselves to differentiate themselves from others in it. The big challenge to this is when you start having a group that fragments and splits every session so that instead of running one game you’re actually running two, three, or more that all are running concurrently at the same table. That gets exhausting quickly no matter how good a GM you are.
Myself, I allow splitting the party, but I usually try to set some soft limits on it so that I’m not juggling too many different split threads from the main group, as well as having a bunch of other players sitting around bored while each player that’s off to do their own thing spends time in the limelight. When you have a table of 6 players and half of them are off doing their own thing for much of a 4 hour game session, the session just doesn’t tend to be as cohesive as a whole, and there isn’t as much fun to be had.