I have 4 sheaths for folders. Between the 4 of them, I can sheath almost every folder I have (from a Leek to a 110), though 1 or 2 are too large for any of the 4.
I prefer to carry a folder in a sheath, even if the knife has a clip. Now, if the knife doesn't have a clip, then it is no contest. I'd rather open a sheath than do digging in my pocket. But even if the knife does have a sheath, I find it more comfortable to have the weight and mass of the knife in it's own exterior pocket than fighting for space with my keys and wallet and such. My pants all wear pretty tight, so pocket space is at a premium.
I do wear some shorts, though, that have no belt loops. In those cases, I really enjoy smaller knives that ride low in the pocket such as my Kershaw Leek. No sheath option when I've got those shorts on.
And I do pocket a Victorinox Super Tinker no matter what other knife I have on me.
But if my pants or shorts have belt loops, I'll go with a sheath.
Edit: I just checked, and even my biggest folders will fit in my biggest folder sheath. And my SAK fits nicely in my smallest sheath. So every knife I own can be sheathed, and if I have a belt on, will be sheathed. But if I am wearing any of my more "laid back" shorts will no belt loops, then a slim folder with a clip is king.