Big yay for disc brakes from me, after about 4000 miles on new bike with them. Allows wider tires on road frame, less work taking wheels off, easier routine maintenance, better stopping on wet roads. On my touring bike (Trek 520), at my weight (230 lbs), stopping on a down hill was always an adventure, even unloaded.
There are a few routes I ride routinely that have turns near the bottom of hills - I can now actually brake with one arm and signal the turn with the other, while on the rim brakes that was not really an option.
Since I generally avoid riding in the rain, and since those times where they give me advantage over my other bikes with rim brakes are probably less than 1% of the time, I would not call them "must have" by any means - but very little of what we spend on bikes is "must have" anyway or we would all be riding the first bike we bought.
On my Domane SL6 disc, the weight penalty was 1 lb - being able to use my preferred 32 mm tires was worth that alone. At less than .5% of my body weight, I can counteract the added weight by having two fewer beers per month...