I can smith my own rules. I probably will smith my own rules. But when I do smith out my own rules, a process that generally involves a bit of sweat and blood, I always have the feeling that I'm doing the work that I paid the guy who wrote the book to do. I really don't want to find huge holes in the rules after I bought a book. And I won't buy a book of which I'm only going to use a half-dozen or so pages out of every hundred. Not a dozen. Not twenty. I'd much rather buy a book in which I'm only not using a half-dozen or so pages out of every hundred.
But beyond those annoyances, there are solid and concrete reasons to have as complete of a gaming system as possible.
The most important reason is the problem of 'out of sight, out of mind'. In my experience, anything that the rules don't cover disappears as an option. A game system that doesn't provide an option explicitly ends up in the minds of most of its players (and referees) forbiding the option. Getting beyond that straight jacket takes alot of work. You have to develop out the rules for everything you want which adds flavor and depth to the campaign and that takes time - time that could go to gaming. You can go with supplements, and sometimes those are really good, but 90% of the time supplemental rules written by amateurs and sem-pros are just lame. There aren't many people out there that can write solid and flavorful RPG rules, and to a certain extent that includes me (although at least I get the exact flavor I want).
This problem extends to the fact that a good rule not only tells a player what he can do, but it gives him insight into the game universe so that he's inspired to create a more interesting character or to at least act in a more interesting way. To use the most trivial example, a combat system that has options for tripping, grappling, pushing, throwing, disarming, sundering, feinting, taking cover, leaping, fighting defensively, gaining the high ground, clinching, circling, lunging, charging, pinning weapons, evading, tackling, and so forth, and makes these options available to the player influences how combat plays out. The player, by virtue of having the options, is inspired to make use of them and thus makes the combat a more interesting story to tell and experience. If the rules for these things aren't provided, they just won't happen and in many campaigns the referee will simply rule that they can't happen.
Any crunch can inspire a player to make a more powerful character. That sort of crunch doesn't impress me in the least. Ideas like that I don't have to pay anyone for.
The sad part of this is that the economics of RPG's encourage companies not to improve and extend thier core into untouched areas, but instead to simply provide more the player with more familiar options.
Now, ideally you have an elegant system in which a few core mechanics cover just about everything. But my personel feeling is that elegance is something that takes time and you won't hit on it on your first attempt. There are no end of systems out there that tried to do everything with a 'single' 'elegant' mechanic and made a mess. Part of this comes from the fact that the universe itself isn't elegant. The real universe is messy. I doubt there is a grand unified rule of gaming that produces good resolution to every situation. Granted, you can resolve everything with a single rule, saying flipping a coin or playing rock/papper/scissors but that's not quite the same thing. You get an answer, but the answer usually doesn't bear much scrutiny. I'd rather the game designers go for comprehensive and then revise themselves into an elegant system as they gain experience with the system. Rome wasn't built in a day. Neither was 3rd edition.