What's scary about predestination is that it can easily be misrepresented if not careful. To be honest I have questioned if I have been predestined especially during difficult times in my personal life etc. I know that was nothing but the enemy trying to convince me that God did not care about me or my situation. I had believed in predestination because it was taught to me but I did not study it for myself. So for the passive Christian the concept of predestination maybe scary if they are not sure of what God actually says about predestination or assurance and justification for that matter.
The most comforting thing about predestination is that two words before predestination appears love appears. See it was "In love He predestinated us . . .", how awesome is that!!!!!!!!!!!!!!!!! To know that God acts out of deep love for you and I is so wonderful and so overwhelming to process at time.
Paul brings this up because the Jewish Christians has a history with God if you will, They were very secure in who they were in God, because they were His chosen people. But the Gentiles on the other hand were just getting to know the only true and living God. I am sure they were also overwhelmed by God's loving kindness and questioned if they really belonged to Him at times. I can imagine them thinking, "God I think you made a mistake my choosing me to be your child." By Paul explaining to the believers in Ephesus that even before God created the world he made the loving decision to include the Gentiles in His family they had to feel a since of confidence in who they were in God and in this plan for salvation.
I think Paul wanted the believers in Ephesus to know that their being predestinated is truly a reason to praise God.