The difference between the flex and non-flex is generally the price of the cancellation fee. I just pay for the lowest, and if i need to change it’s $100, or $150 for cancel.
If you were putting in placeholders, not intending to fly, and change for something else later, more than once, the flex might be an option.
Oh, I paid for the full flex once, as full flex got you access to the AC Signature Lounge with private dining. Unfortunately, i also cancelled - free of charge! - to take a last trip AA first across the atlantic and their flagship dining.