Flexible tickets are worth it if and only if it's highly likely you will need to change your tickets at short notice.
If you're on a business trip of uncertain length (say, negotiating a contract), this kind of thing is frequent, and a flexible ticket that costs $1000 is a better deal than a fixed ticket that costs $500 plus >50% odds of needing to pay a $2000 walk-up fare — especially if that negotiation drags on and you end up changing your reservation two, three or four times. One of my previous employers mandated that all tickets purchased in advance be flexible for this very reason.
If you're a backpacker, on the other hand, this is unlikely to ever be the case, and it'll be much cheaper to buy fixed tickets and work your own schedule around the fixed dates. For your cheap ticket, the "change fee" is in addition to the fare difference, so the total cost with a last-minute change would sum up to $500 (original ticket) plus $125 (change fee) plus the difference between the original price and the walk-up fare (potentially >$1000).
Note that the conditions of flexible tickets vary, and many do charge the fare difference as well. However, since flexi fares are more expensive in the first place, on busy routes with some spare capacity the fare difference to the cheapest available seat is often zero.