If you turn off the automatic relist option you're just while wasting what little promotions eBay gives you. With 40 free listings per month on the most expensive eBay site in the world (when it comes to insertion fees on regular non-store accounts), you will want every single free relisting option you can get when things don't sell straight away.

The Good 'til Cancelled option does not allow an item to relist automatically, it will always use up a listing every single month. GTC is only useful to sellers who ended up in eBay hell and do not get the option to relist whatsoever.