I would look for and ask somebody who has actually used a shotgun in a fight or potentially lethal confrontation, and ask them if they thought they had enough rounds in the gun, or if they wanted the ability to reload with rounds on the gun.
It also depends on your intended use of your shotgun - if it is for home defense, and I will assume it is, then it will likely be a grab-and-go situation, without much time or opportunity to put on a vest or bandoleer or grab some spare ammo. The persons I have talked to have said that in this type of an emergency situation the ammo you are likely to have available will be what's in or on the gun, or what is on you.
As to other's comments about balance, that is a personal and training issue. I have shot my 870's and Benelli M1 Super 90's a lot, both with and without a sidesaddle, and the balance is not an issue for me. We aren't talking about shooting clay targets or dove hunting - that is what the o/u is for.
I'd buy one for the $20 or so bucks they cost, shoot up a case of ammo, practice reloading from the sidesaddle, and then decide.
Another point not raised is putting extra rounds on the buttstock as an alternative - Spectre (sp?) gear makes a great carrier that doesn't slide around on the stock.
As for me, I'd rather have too many rounds than not enough, but that is just my opinion, as I've never been in a fight with a shotgun.