Define "Combat"....
Are you talking about a weapon for a point man leading the squad down a jungle trail?
Or, something for a reservist called up on active duty to use out of a firing port of the APC he/she's driving?
Or, something for a civilian who lives too close to the werewolves, has little`experience with military style weaponry, but a clear and pressing need for a shoulder arm of great short range power?
Or a small business owner, in an area likely to go off the next time someone gets beaten up on videotape by the police?
Personally, in any of the above scenarios, I'd be content with one of my 870s, but others may differ.
As to why, after 40 years of use ranging from goose and duck hunting to line of duty, after taking a truckload or so of game,after busting a few thousand clay pigeons and trying to bust many thousand more, after protecting myself and family, I know that an 870(in trained and motivated hands)comes as close to 100% reliability as anything made by humans.
Any reliable shotgun is a great defensive tool. The 870s leads them all in reliability...