Merits?
1) Simplicity. The only thing simpler is a single shot. Easy to use, even for a non 'gun person,' as an illustration of this my sainted mother's 'house gun' for decades was a cut- down 20 ga. double barrel, a 311 with Sears' name on it. She now has a Beretta CX4 Storm carbine in 9mm, as the shotgun's recoil finally got to be more than she could handle. But the little double worked for her until she was 79.
2) Safety. Carry it broken open and empty and all the world can see that gun is safe. Easiest off all designs to clear and reload is the break-open gun, in the field when crossing fences and other obstacles this gets to be a big advantage after a while.
3) Practicality. Two shots is plenty for sporting situations. And a proper double gives you a choice of two chokes instantly as needed for different ranges. It's essentially two guns side by side. And it's useful in "social work" as well.
4) Tradition. It's just "right" to hunt birds with a SXS. Your elders will approve, your mentors will feel the time they invested in you was not wasted.
5) Classic- there is no more beautiful shotgun than a good SXS. It doesn't have to be a bespoke London gun either.
So what are you waiting for?
lpl/nc 8^)
