Just a personal preference.....rigid kydex would negate such a need but leather IWB w/o a thumbreak have produced some exciting results for office weenies and drivers who wear such sitting in seats w/ arms on em or high center consoles has produced some unintended noise. 3 incident in my old department convinced me to not use a leather rig w/o a retaining thumbreak or jordan style security etc ......
In belt carry I prefer a Bianchi black widow with thumb break for my Glocks.