The big advantage of the shotgun is it's adaptability.
Over the last 3 years, I lived in 3 different places.
As I moved, HOW my 870P Police gun was set up changed.
#1 was a house in a small town, with plenty of light, a police station literally 1 block away, and no heavy-duty crime. I figured there would be ONE Bad Guy who would arrive on foot, and who wouldn't stay to fight.
The gun at that time had NO extension, NO light, and was loaded with #1 buckshot.
We moved in to a mobile home while we waited for an apartment to be readied, so the gun changed.
Now we were 10 miles out of town, law enforcement was the Sheriff who was 20 minutes away, there was NO light, and NO neighbors.
The gun was changed to a magazine extension "Dutch loaded" with a mix of slugs, 00 buckshot and #1 buckshot.
I had a butt cuff with spare ammo, and a spare ammo bag with a strong light in it.
I figured Bad Guys would arrive in a car, there would be at least two, a fight would start off close and then open up to everyone behind cover, and there would be no help for some time.
After a couple of months, the new apartment was ready, and we moved.
Again the gun changed.
Now we're in another, larger town.
There is a nearby crime problem, but police are CLOSE by and VERY active.
There's PLENTY of light, and very close neighbors, including a school for handicapped children.
I figure there will be at most 2 Bad Guys and these types don't stay to fight.
Police can actually hear shooting and respond in seconds.
Now the gun has a standard magazine, no light, and is loaded with reduced recoil #4 buckshot.
In this case, due to the tighter quarters, speed with the gun is more important than magazine capacity.
So, even though a magazine extension is nice to have, and can be very important in some circumstances, it's NOT essential.
The key to having a good setup on a defense shotgun is to do a "Real World"evaluation of your particular situation, then do a "cost-benefit" analysis of just what accessories are really needed, or even advisable.
We'd all like to have a Remington 870 Super Magnum loaded with 10 rounds of 3 1/2" #00 buckshot and an electronic dot sight, but this may not be what's needed, and may actually be a hazard to the actual threat situation.
If you're likely to be facing an enraged biker gang with no help available, maybe you need an extension, but don't set your gun up for something that's unlikely to happen, when the actual threat may be a single burglar who's surprised to find the owner home.
Sit down and give some thought to what's the MOST likely problem you might face, then set your gun up to meet that threat.
Too many people way overload the gun and slow it down to meet fantasy situations they're very unlikely to see, and wind up with a gun so overloaded and tricked out, it's too slow to meet the real threat.