I don't disagree that overpenetration should be a concern for a lot of people. It isn't for me, so I like the AR15 for home defense. I've got a cabin that is even more remote, and comes replete with occasional threats from various wildlife, and I like it there too -- but that is with the knowledge that the nearest human being should be no closer than 50 acres away.
Regarding the ammo, for a small caliber weapon, and the accuracy of the average person who shoots, your odds of stopping an assailant with 10 rounds is pretty good, at about 85.1%. If there are multiple assailants though, 10 rounds doesn't provide particularly great odds - at 35%. 12 rounds gets you 50% odds against 2 assailants.
The main reason I argue against ammunition capacity restrictions for citizens though, is that generally speaking, criminals have the advantage of preparation in their crime, and aren't going to respect magazine limitations -- especially those of the more ludicrous nature like found in New York, where there is a 7 round limit. You can have a larger than 7 round magazine, but may only fill it with 7 rounds. No criminal preparing to engage in criminal activities is going to observe that restriction, while there may well be law abiding folks naive enough to obey such a law and find themselves out of ammunition when they need it.
Further, according to the Supreme Court, bans on arms "in common use" are effectively unconstitutional, and banning down to a 7 round capacity limit is a violation of that decision. As you said, there are numerous handguns containing 10 or more rounds, with Glocks generally fairing on the higher side of capacity, and older weapons like the 1911 being 7 or 8+1, but standard capacities are usually somewhere between 12 and 18 rounds on the most common of firearms. For the AR15, 20 rounds is the norm, and as such, should be protected.
Per Alan Gura (the attorney of record for DC v Heller and Chicago v McDonald), you can't make what is "in common use" a result of a restriction either, so that they can't ban all but five found magazines, then rule that as a result of that ban, a five round magazine is what's protected as common use.