At times, thermometers are the best of things... or the worst of things.
If you learn how to cook without a thermometer, either on the pit or to temp the meat,.... you actually learn more about fire control, and you learn to watch the meat, listen to it, poke it, reposition it, when to baste it, etc. The best thing is.... you can usually cook average to above average BBQ all the time, and you are at ease on most any cooker. The worst thing is.... this takes a long time to master.
If you learn to cook using thermometers you try to mirror the techniques you read about or see used on TV. This leads to sometimes unnecessarily fretting because you can't hold a 225° for 4 hours, or maybe you overshoot a 275° and get freaked out if the pit is 280°. Some people will even start doubting their cooker because it does not behave exactly like someone else's does. The best thing is, when everything is dialed in and working right... you can turn out very consistent BBQ. Worst thing is... if a therm fails, some cooks panic.
Until 10 or 12 years ago when I bought my first BGE, I never cooked on a pit that had a thermometer, and the only time I would use a meat thermometer (the old stick-and-stay kind) was when cooking a prime rib. That said, I got hooked on cable thermometers and the instant read ones from Thermoworks and use thermometers to make my cooking a more relaxed and pleasant experience.