I know. You can adjust where they're positioned before the game but not where you want them to go in game. In the fluid formation system, you can instruct RB to be a CM while the team is on the ball/attacking. When you're team is on the ball, you can have 4231, when you don't have the ball 4141. It's more in-depth than picking a formation and giving players specific instructions etc. http://youtu.be/NHTrBUJNgPA?t=3m33s
Custom tactics in FIFA has been there for years. But it's not as intuitive and as easy as how this was presented now in PES. You can adjust team width, pressure, etc in the custom tactics panel.