1. Five out allows you to play everyone in the post and exploit match ups.
With the five out, any player with a good match up in the post can cut to the basket and post up on the block to take advantage. Your point guard might have a small defender on him and in a 3 out it's hard to be able to duck him in and look to hit him on the post up. In a five out however it's a simple part of the offense: pass-basket cut- post up. You can also use it to attack a star perimeter player who really doesn't want to play defense that hard. It's easy if he stays on the perimeter, he can sag off a little and has help, but if he's being aggressively posted up it takes him out of his comfort zone. It also has the potential to cause him to foul. This type of spacing allows any player on the floor to be a post threat at any time.
2. Five out allows you to be flexible with how your post player gets touches, it's easy to guard someone standing in the lane.
In the 3 and 4 out your post is basically block to block or block, to high post, to block. This is fine but it becomes pretty easy to guard a player that is inside, especially with the help defenders added in a 3 out. It's harder for a defender to defend the post when the post is curl cutting off a screen, screening and then diving to the rim, backscreening and shaping to the high post, passing and cutting, backdoor cutting, etc. Guarding all those actions becomes a much more difficult task that guarding a player standing on the block.
Something we did at St. Croix Prep was have our "post" players hold for a pass or two if they were being fronted on the initial cut. It gave us high low and seal looks you would get in a four out.
3. Having permanent post players (especially 3-out) clogs up the lane for cutters and drivers.
I am just not sure how your perimeter players are expected to score inside in a 3 out. Watch teams that play a 3 out or a high low motion. The guards cut behind the posts so the post and their defenders block the pass to the cutter. When they screen it's a straight cut and the screener flairs out. What you end up with is passing the ball back and fourth around the perimeter and not really looking to attack. It's much better with the four out, but that post is still there - although it opens the floor for drive and dump, it still makes it harder to pass and cut cleanly.
4. It allows you to train your players to be complete players.
I am a huge fan of the European approach to skill development. Bigs should be able to dribble, pass, and shoot and guards should know their away around the block. Everyone should do everything. It allows you to be more flexible in your attack and really key in on mismatches. Even Michael Jordan, a great perimeter player, had a post up game when he needed it. How deadly was Charles Barkley? Put your power forward on him Barkley is taking him to the rack and shooting threes over him. Put your guard on him and he’s taking him inside and posting him up. Same with Bird, Magic, etc. Playing a five out allows you to develop players in this fashion.
5. It allows you to play your more skilled and effective players, not put a player on the floor to fill a position.
How many times have you seen a team that plays a less skilled/talented player at the 5 because he's bigger while at the same time there may be a shorter player on the bench who plays the "4"? This player may be more effective in the game (even rebounding and defense) but they don't start him because he's not a "five". Why not play the more effective player? The five out allows you to put your best talent on the floor.
Also, I've seen teams with skilled bigs who can pass and shoot that get stuck on the block because they are tall. Once saw a kid on the block who was 6-6 but could shoot well from the perimeter - why not put him out there, draw the defense, and open up your game?!
6. It allows an undersized post player to compete and thrive.
If I had a legitimate, dominant post player who was 6-9+ I would be playing a 4 out. But most of us are not blessed with these players on a regular basis – if ever. Our players are undersized post players. When I was in Iowa my tallest starter was 6-2 and was our best overall player - could pass, shoot, dribble. If I ran a 3 our 4 out he would be in the post - but why?! As good as he was I doubt he would consistently win many battles against the 6-5 to 6-8 post players we faced on a nightly basis. In the five out he could cut in and post from time to time but also had the luxury of taking the bigger, slower player he was facing off the bounce or shooting when they played off of him.
Gives offensive players a better chance at an offensive rebound as well because they have more space to evade the block out. It’s harder to block out a perimeter player than a player on the block.
Your undersized post player has a better chance of getting the ball by cutting PAST a bigger defender to the basket than he does standing and battling it out on the block. Cutting also allows him to get the position he wants.
7. It stretches your help.
Like it or not, no matter how much we drill, most high school players are mediocre help defenders at best. Playing a five out, players naturally hug their man a little bit tighter. For instance, the ball is on the right wing. The player guarding the left corner man will be on the block, not mid lane like good help. If that player was on the block, there is a better chance they'd be comfortable enough to play mid-lane they feel more comfortable selling out with their man closer to them. The stretched help defender allows your cutters to cut in front of the help when they cut to the rim - not into it or behind the help. It also makes the defense one step slower helping on the drive.
8. It makes an opponent's defender play out of position.
In high school basketball, many 5s are that traditional football player, usually a lineman - big, bulky, slow. The type of player who is used to throwing his weight around under the rim. Why not bring him outside? Make him defend cutters, defend the drive, defend screens, and have to actually find a guy, sprint at him, and box him out? It's harder to rebound when guarding a perimeter player - especially when you have to sprint from the help position. Making that big play on the perimeter takes him out of his element and makes him do things he is not comfortable with - and frankly may not be able to do.
In closing, I'm not saying that the 4 our 3 out is a bad thing - although I've become much less of a fan of three out over the years for spacing reasons. What I merely saying is that you don't need to have a post in the traditional sense of the word to be successful. In fact