For me it's not a matter of practicality; I just happen to prefer the style. And I agree that I'd be too worried about flashing my skivvies in the likes of a Utilikilt or 511 (again, not to mention the inner edge getting on my nerves as I walk). But the medium-apron style, which is about 1/3 of the circumference, has never given me that problem. Literally the only time my modesty has been compromised was once when I was "Marilyn Monroed" by a really strong gust of wind...and that was the pleats, not the apron.
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Here you can clearly see the difference. On the left is a Utilikilt, while on the right is a utility kilt from Celtic Croft in the same overall style as the UTs that liammaclean is talking about.