What are the pros and cons of practising figure-eight landings on perpendicular runways?
There's nothing stopping you from doing this at an untowered airfield, at least in the US, UK and most of the places I've flown to. There just isn't any benefit. What you should be looking to do is keep up the practice you need to fly safely, which is the spirit of the law rather than the letter of the law which is to maintain currency within regulation. 

Could you fly some weird-ass pattern and save about 5 bucks? Sure! Does cutting corners make you a pilot I'd fly with? Absolutely not, because you are looking for ways to economize rather than fly diligently. Flying the pattern is good because it puts you where other pilots expect you and keeps you out of conflict, it also means you know what to do when you arrive at an airfield for an approach. If you're constantly flying some strange bent figure-eight you're bound to mess it up and that could get you into trouble. Take-offs and landings are the busiest and most dangerous times when flying, the whole point is to standardize and reduce risk, if you are flying to a different runway every time the approach constantly changes which isn't good. A good landing comes from a good approach, which starts from a consistent circuit. 

Flying the same pattern is also good for your practice and improving your technique. Flying repeated patterns with the same wind conditions means you practice in those conditions until you nail them. It allows you to hone your technique. If you have a different wind, runway and visual cues every few minutes you don't get the consistency to learn what you're doing wrong and improve it.