This has to be alone the line of Celtic's other Ramblings that Rhoose and other airports should become theme parks and amusement arcades for juveniles that have got nothing better to do:
CelticRambler
It makes commercial sense to have it free 24 hours a day and do everything possible to make the airport the hottest spot in South Wales. What could possibly be better than making the landside part of the terminal a place that (young) people really want to be and be seen? Make sure they've got easy access on public transport, great (cheap) coffee and snacks and cool spaces to hang out.
Advantage one: extra footfall for the other retailers; advantage two: brand awareness for any airline associated with the terminal; advantage three: airport makes money from non-flying passengers and can indulge in commercial innovation, for example no landing charges for commercial flights <20 seats and, what the heck, maybe pay the APD too.
Make it into a place worth visiting in its own right, then you'll start getting boyfriends surprising their girlfriends, or parents surprising their children, by surprising them with "oh, actually we're not stopping today, you're flying to [Paris/Rome/NewYork] and now "Cardiff" gets a reputation for being even more of a fun place and people start coming from the other side of the Severn.
All for the price of free WiFi and pulling a few heads out of the sand.