Let me start by saying that most of the guns that I own and use on a regular basis have a magazine capacity of less than 10 rounds and that for most of my needs, that is plenty. I do, however, own a couple of guns that have "high-capacity" magazines (CZ-75 and 922r-converted Saiga .223) and, under certain circumstances, they can be my preferred firearms.
While I live in a rather low-risk area and try to live a low-risk lifestyle, I do sometimes have to venture into areas that are not so low-risk. In certain parts of large cities where gang-activity is not uncommon, the 17+1 that my CZ-75 holds is much more comforting than my revolvers or single-stack semi-autos.
Likewise, the Saiga is my dual-purpose home defense/wait-out-the-crisis rifle. Four years ago, the town in which I live flooded and much of it was cut off from the outside world for a couple of days. While the police, fire department, hospital, and even National Guard (there is a large military base just to the north) did the best they could, the flood happened so fast and was so devastating that they were simply overwhelmed and we did have some looting problems (though thankfully there were no acts of violence). This made me think that, should a similar natural disaster occur and take more than a couple of days to resolve (such as what we saw with Hurrican Katrina), help might be simply unable to reach me and my family and a high-capacity firearm would be a very nice thing to have should I need to defend myself for an indefinite amount of time.
More to the point, however, my ownership of high-capacity magazines and firearms hurts no one that doesn't already intend to do evil upon me and my loved ones. That being the case, I fail to see why someone who does not know me, my situation, or my wants and needs should get to tell me how many rounds my gun can hold.
Even more to the point, I've watched the tactics of gun-banners long enough to realize that they would not be satisfied simply by banning high-capacity magazines. Various figures within the anti-gun movement have admitted over the years that their ultimate goal is a total ban of all privately owned firearms (such as Dianne Feinstein's infamous "Turn them all in Mr. and Mrs. America" comment), but because that's too big a pill to swallow at once they instead try to chip away at our rights a little at a time. The goal of this strategy is to zero in on small minorities within the gun-owning community thereby dwindling its numbers until the gun-owning community itself is a small minority. At that point, the gun-owning community would be too small to stop whatever draconian regulations the gun-banners wish to impose upon them. To quote Benjamin Franklin, "we must all hang together or we will assuredly hang separately."