This post and its continuation shed some light on why keeping character and player knowledge entirely seperate does little to keep the party together. Basically, since the players don’t want the party to split up and all lose spotlight time, each character needs a reason to stay with the group. This tends to lead to the perennial question “Why am I going with you this time?” which becomes increasingly difficult to answer. There are a few common solutions. One possibility is to allow pawn stance play, in which the character does what the player wants (i.e. stays with the party) whether or not it is plausible based on the character. With this solution, you lose strong character motivations. Another is to provide strong party based reasons to stay together, such as all characters working for the same employer who sends them on missions together. With this solution, it becomes much harder to work in personal issues. In both cases, you lose something in order to achieve party unity.
In the linked post, Mike Holmes describes a third option, which is to allow author stance – the use of player knowledge to affect character actions, but with in character plausibility/justification. With this stance allowed, the concept of “party” becomes much less important, and splitting up doesn’t have the effect of reducing spotlight time. This is because players can plausibly involve themselves in interesting scenes (that the player knows about but the character doesn’t) whenever they choose.