I would say it goes on a cline, that each additional inch of reach advantage you have over your opponent has additional functionality. E.g. a 1 inch reach advantage over your opponent might be worth 1x reachbucks, whereas a 3 inch advantage is worth 3x reachbucks, or 10 inch reach advantage is worth 10x reachbucks. So in answer to your question, I think even the smallest reach advantage is still some advantage, but it is hard to put it in words exactly.
Having trained muay thai casually over the years, and can just mention some of my insight. Dudes with noticeably less reach you can keep them at bay with your jab in their face constantly. I am Aussie but lived in Korea and China a few years. Training with smaller dudes a lot of the time who had less reach, when we were sparring their ability to get into range was extremely limited, by peppering them with the jab constantly.
I would just mention that one time I remember sparring an older Chinese dude, who moved around so quickly it neutralised the jab them at bay strategy, and in effect he was so fast that I had no ability to touch him and he kept darting in taking pot shots at me and there was nothing I could do about it. True I am just a casual, but it's kind of interesting how a smaller dude can neutralise the size/reach disadvantage through speed. I remember it being insanely frustrating that despite being much larger, with longer reach, I could not touch the dude to save my life.
In MMA I believe the ability to mix in take downs or engage in grappling neutralises the reach advantage more than in Kickboxing/Muay Thai.