Isn't it refundable only if you have not used the ticket? Once you checked in, you only get a portion refunded, right? What would happen if you flew the outbound and decide not to come back. Will they refund you half the ticket price?

If you have not flown, then your ticket is still completely refundable, regardless of whether you have checked in or not.

If you fly one or more segments, then the remaining value of your ticket is refunded, if you so choose. It could be half, but it depends on how your ticket was originally priced.

A few years ago, BOS had an AA Flagship Lounge that could be used by top-tier (Executive Platinum) flyers on international itineraries even if their next flight on that itinerary was domestic. There was an EXP consultant, a frequent poster here once, who flew a lot of trans-cons. He'd buy refundable tickets from his California destination to somewhere in Asia so he could claim to be on an international trip, then cash them in. On the return, he'd do the same with a ticket from Boston to London or Paris.

AA was onto his game, everyone on the EXP desk knew about it, but his business (typically paid F domestically with a trans-con every few weeks, plus some real international paid J or F) was worth so much that they let him keep playing. Didn't cost them much, and it kept a valuable customer happy.

I had thought of this as something that someone might do on an airline that they rarely flew and for which it wasn't worth buying a lounge membership.

(For example, an AA flyer on what they anticipate would be their only UA roundtrip of the year. If one assumes a connecting flight, it would cost $200 per person to buy passes to the RCC at each stop on the itin.)

It hadn't occurred to me that someone would do it on an airline they regularly flew even if the airline didn't care. There's some point after which the hassle of doing this would outweigh the cost of just joining your regular airline's lounge.