I've struggled with this for a while...namely because I've never found a retention holster that I really like (except for the 5.11 holster for my G34, but I don't shoot glock anymore)
For me now, at rigorous matches i tend to use a retention holster....but for local matches with limited movement, I use my bladetech.
Non-retention holsters can have significant retention....with mine, I have two thumbscrews on mine and if the stage calls for it, I tighten them right now