And there is effect of becoming oversensitive when encountering strong stressor randomly and often. I stead of getting less sensitive people can start reacting more aggressively at smaller threat.

Your train of thought here reflects classic avoidant behavior -- it's not therapeutic unless I'm in control of all the variables. Certainly you're correct depending on the type of fear being extinguished -- dropping buckets of spiders randomly on someone afraid of spiders isn't likely to help things at all, nor is dropping someone with GAD in the middle of a crowded concert -- but for the particular phobia (social anxiety) and exposure (small talk with strangers in public) we're talking about in this thread, it's definitely the low-stakes kind of exposure exposure therapy aims to provide.