If, for example you book a ticket with a $200 change fee and find you need to change your flight the day before you travel. In this case, you might need to pay the change fee AND the new flight has no availability in the lower fare buckets any more - in this case you end up paying for the new flight, plus the change fee, which could end up costing more than the flex ticket in the first place

If you're booking a long way out and know that you are very unlikely to be changing your flight then of course you can choose to buy a ticket which charges for changes (or even sometimes doesn't allow any changes at all), but you need to check the fare details carefully - some fares will reprice based on the fares available on the day you originally booked, but many will reprice based on the fares available on the day you make the change... these costs can be significant.