One time I tried experimenting with putting side slash pockets in a kilt I was making and I did not like the results. 1) When I put anything with any size in the pockets, it created a bulge in my kilt. 2) I didn't really use the pockets much because I could carry anything I needed in my sporran.

I've seen Steve's kilts in person and he has magically discovered a way to create "pockets of holding" in his kilts so that they look empty even when you have stuffed 40 lbs of gear in them. No idea how he does it. But I'd say unless you are buying a Freedom Kilt, expect to see some bulging in the shape of the kilt when you stuff your pockets.

I have also seen several traditional kilts with small pockets sewn into the center front of the inside apron (where the sporran would hang). These could be quite useful for members of bands or other groups who are required to wear parade style hair sporrans, many of which are made with no functional pouches (or pouches of minimum size). This way you can keep your ID and some cash on your person when in uniform.

However, for general wear I don't see the purpose for this kind of pocket. It doesn't hold that much, it's right where your sporran hangs anyway, and if you ever need to access it, it requires you to stick your hand under your apron and dig around.... well, it just doesn't look "dignified"! :-)

I have found a solution to all of this, happily! I have a small collection of detachable pockets that can be worn suspended from a belt with any of my kilts. This way I can mix and match, and wear them with different kilts. Some of these detachable pockets have fur, some are plain leather. You can get them with metal cantles, tassels, and all kinds of other options. You can have them made in formal or casual styles. Really adds a lot to your wardrobe. And they even have a special Gaelic name: sporran. I like mine so much I never wear a kilt without one! :-)