I had a chance to sit down and talk with an officer who was shot in the upper right torso with a load of birdshot from 15' (no body armor - off duty). It left a large ugly wound; but because he was partially turned when it hit, his arm took a lot of the blast and he was left with a really gruesome looking wound with few pellets penetrating to the torso and none hitting anything vital. So gruesome looking that when he walked over to a neighbor's house to ask them to call 911, his neighbor freaked out and he made the call himself. It was a wake up call to me that such a thing could happen and as I read more on the subject, I understood how.
If I thought someone was an immediate threat of death or serious bodily injury to myself or others, I think I would be concerned about using a round that left them physically able to move and communicate for 15 minutes or so.
Additionally, with birdshot, a lot of things that are only concealment from heavier shot or pistol/rifle rounds become hard cover (refrigerators, bookcases, interior walls, down or leather jackets). I would hate to be in a gunfight inside my own home where my rounds can't penetrate interior walls or furniture but the home invader's rounds can. I doubt the home invader is all that concerned about overpenetration or a safe background, so the more shooting he does, the bigger problem there is.