While I'm not quite ready to speak quite so ... derisively ... as Drail, I'm not ready to grant holster clips much legitimacy. They fall into the category of things that work ok, sometimes, usually, pretty much -- and lure people into thinking that they will work correctly always, as needed, on demand, and when the situation is more critical.
The fact that they tend to come on cheapy, bad, "universal-fit," nylon holsters -- and DON'T come on high-quality, well-fit, custom and/or high-performance leather or Kydex carry and duty holsters -- can be a bit of a clue. Makers and manufacturers don't put these on good, reliable, secure, long-lasting holsters because they'd be an obvious weak link.
They really only have a place on holsters where "cheap" is the most important factor.