While there are players that can successfully play while not reading posts unrelated to themselves for the sake of keeping their character(s) truly in ignorance of things not directly pertaining to them, these players are exceedingly rare.

The reasons for not reading posts can be many, but in general they are typically rather weak. Some examples we've come across:
[list][*]Player wants to keep his/her PC in ignorance of unobserved happenings.
[*]Player doesn't like "romance" (i.e. interactions that could in theory lead two characters in a group RP to head towards a black-out scene).
[*]Player dislikes one of the other players (why would they be in an RP with someone like that?).
[*]some posts are "Too long".
[/list]

there are probably others too, but we can't remember them right now. The general tendency is that it leads to grievous mistakes in the IC. Either due to ignoring one post too many, or through not having read posts properly (the ones read). In any case, its something we dislike in players, but we can't exactly force players to actually read. Sure, demands can be made, but they can't truly be enforced.

If you do plan not to read everything, then you should make sure to have good communication with the others so that you can get the others to help you avoid errors in your writing due to not having read everything.