Second Clip/attaching point = more stability. Helps hold the grip at the same angle and at the same place on the belt. Distributes the weight and "tug" on the belt over a wider area. More of an issue with heavier, longer carry pieces than with lighter, shorter ones. A double-stack .45 (for instance) could best be carried with dual attachment points.
Single clip attached over the center of the carry piece makes for a bit thicker "lump" under your belt, all else being equal. That said, the thin, light single-stack .380 (or even 9mm) might be perfectly suited to carry with a single-attachment holster.
Agreed!
To all, single clip holsters tend to rock back and forth, and, if you get a cheaper one, the clip might break, especially if it's one of those cheaper "plastic" clips (referring to Kydex, polymers, etc). I had that happens with a holster that uses a Kydex clip. Metal is better. Also, use a little Loktite to hold the screw in place so that it won't back out. I had that happen once...