Two notes from my experience doing this:
1) If you buy a refundable fare you can cancel it and request a refund anytime within the ticket validity period (1 yr from purchase) including after the flight it was issued for departs. For non-refundable tickets this is not the case.

2) Technically you should have a valid ticket when you are beyond the TSA checkpoint so, depending of how rules focused you are, you may want to wait until you are out of the secure area before refunding.

While I would expect DL to eventually cramp down on someone who takes frequent advantage of the 24-hr refund option (they don’t technically have to do it if you buy less than one week before departure) a refundable fare is just that and I doubt you’d get in trouble for buy and refunding such a ticket frequently.

Also, IME, refunding a refundable ticket usually takes a few days to post not a few weeks. So if you have a credit card, it shouldn’t matter the price of the refundable ticket and you are less likely to screw it up and end up holding the bag on a ticket you never planned to use.