I should clairify what i mean on ' blocking' a players POV.

it doesn't always mean placing a physical block in the way, ( but thats a normal way), but also means channeling the players POV.

A 'normal' player will instictively turn away from natural bariers if they know they cannot go over /through them to find way around that obstical. like a mountain or a wall, so they will turn thier POV in order to navigate around it. So to reduce clipping problems ( low frame rate) you spread your entities out so not too many are in a single given view angle then direct the player to not be looking in one direction for long by controling the flow of direction(s) they travel.

Pick up most first person shooter games for example. in many of the more memerable ones here are things you see ( or actualy don't see)

viewing from one room to the next is limited. ( normaly cannot see beyond 3 rooms in a row stand in room x, see into y, and see the entrance area to z)
transitions from one room to the next usualy don't face eachother
to restrict flow through views

large open area usualy don't have too many moving things in them and many larger rooms are broken up with smaller objectc the player has to navigate through. so making the player change pov allot to get around

enemies usualy arn't thrown at you in a BIG cluster but waves in larger fights.
though may not seem like a wave 1 attacks then another, then 1 dies another is brought in and so on. reduces number on screen at any given time but in the end same effect

many transitions in a level up and down block allot of visual range front back, left right

BTW some MMORPGs are bad examples of clipping managment, or you could say good examples of clipping missmanagment. they try to cram too much into any pov, especialy animations which ultimatly kills game play. ( if you ever played EQ ruins of Kunark aroudn the newbie areas, or did an RVR or dragon raid in DAoC then you know. way too much then the engine can handle)