Macwages, I am sure you have picked up by now that there are no rules, only personal preferences. Given that, I normally wear a sporran with any kilt that does not have pockets. Simply because I need someplace to carry at least a wallet and set of car keys. I have the standard cheap hard leather black one that everyone seems to sell, and so every new kilt wearer ends up buying because they need one and the price is right. Not really very stylish or spectacular, and it usually comes with a chain instead of a strap in case you have to attend an emergency black tie affair! But it is barely large enough to hold a wallet and some keys. I also have the SWK nylon black sporran and love it. Very practical for modern times while having a look that honors the traditional. I also have a "casual" brown leather sporrna that is far superior to my first cheap one and I am very fond of it. It does not have the compartments of the SWK, but is still very nice. There is a pic below. I have also worn fanny packs/bum bags and they work fine as well, although I find them a more casual look.

If the kilt I am wearing has actual pockets, at least a rear hip pocket for a wallet, I do not wear a sporran. I suppose I could, especially with my PK's, but with a rear pocket, 2 side slash pockets, and the underapron pocekt, it seems superfluous. For me, the sporran is a practical answer to no pockets in the kilt, not something that makes the kilt or is a required fashion accessory.

My plain, brown, casual sporran: