People have hit on the two major reasons: 1) Reliability. We can think of all kinds of hypotheticals where one identically ejects a mag. Further, certain malfunctions typically require stripping the mag, its not unheard of for someone to bungle that and drop a mag. It could also just be your very very very unlucky day where you mag its self has some kind of issue. 2) You can not assure your gunfight is going to be the typical 3 round affair. Larry Corriea wrote a great article entitled something lime "my gun fight." The point of the article is that far too often people seem to envision how they will use their gun in SD or how a SD shooting will go down and prepare and focus on that to the exclusion of many other scenarios that are just as likely.
Downside is the extra size and weight. It's easier to conceal a spare mag or even two than it is to conceal a second gun.
Depends on the gun and the mags. I find concealing something like an LCP to be no more difficult than carrying a spare 15-17 round glock mag. In fact I think it is often a little easier and more comfortable. Of course having 7 rounds of 380 in an LCP isn't the same as having 17 more rounds of 9mm for my primary. The same is true for a PM9 sized gun. Carrying a 17 round mag is of course easier than carrying G17 or even a G26, or even a Kahr P9 sized gun.
As has been eluded to, a BUG offers some advantages a spar mag does not. Namely having a complete gun in a different spot from ones primary. I don't see a BUG necessarily being a mutually exclusive to a spar mag. Depending on what is carried as a BUG and how a spar mag is often a better solution to the problem at hand (i.e. a speed reload may be faster than going for the BUG).
One will see people carrying thirty or more rounds in LEO circles and in competition. I know of no one who carries that much for self defense.
I find that a bit surprising. Most of the G19 sized guns hold at least 15 (+1) rounds. A spare mag puts one at thirty. I know a lot of people who carry that sized gun plus a reload.
What I find curious is that when it comes to carrying a gun people seem very quick to label people a "level" above them (i.e. carrying more than what they do) as paranoid, unrealistic, etc, etc. They often do this for people a level below, but instead label them under armed, under prepared, etc. What's interesting to me is you see this everywhere on the spectrum from someone who carries no gun thinking a CCW is a loon (after all how likley are you to every really need that gun), to the 5 shot revolver guy thinking someone with a wonder 9 is over the top to the wonder nine guy thinking someone with a BUG or a reload is going overboard. While each of those people often thinks the one below them is flirting with disaster should Murphy come knocking.
What's more interesting to me is all the things many gun carrying people don't carry or keep with them in their cars, i.e. basic emergency supplies, basic tools, etc. But that's another thread.
