Recent editions of D&D, so I've read, have virtually made a fetish of balance. The word does have a certain warm fuzziness if you don't think too deeply about the implications. When I do, though, it's hard to imagine a greater travesty to inflict upon the game.
If you never face anything but "level appropriate" challenges, what's the point of leveling up at all? You're just on a level treadmill. Every time you become more powerful, your opponents match you exactly. The term "skill tax" was coined to describe a situation in which you have to improve a given skill just to stay even with an increasing challenge level. Well, isn't fetishizing the balanced encounter really just a level tax?
If you never have to run from a hill giant when you're second level, can you fully experience the sense of accomplishment when you defeat him at fifth level?
Is there not a certain feeling of triumph in biting off more than you can chew, retreating, and then returning later with bigger teeth to claim the victory that eluded you before?
Isn't it more suspenseful if that temple of evil has been there the whole time, leaving you to judge for yourself whether you're ready for it? Simply having it pop up when the DM deems you ready doesn't cultivate that same delicious sense of possibility mingled with dread.
If all the kobolds disappear just because your character gains a level, don't you miss out on the opportunity to revel in your newly acquired badassery? A one-sided victory every now and then can be fun.
On the other hand, if you're only allowed to face a "level appropriate" number of kobolds, you never get to experience that "Uh oh!" moment when you realize that in large numbers, they can be pretty damn scary.
How do you even attain balance without nullifying the wonderful wild card of zany and unexpected strategies that can turn a pitched battle into a rout or snatch victory from the jaws of certain defeat? Why would any DM want to deprive his or her players of that thrill?
If every encounter is balanced to be winnable by combat, where's the incentive to sneak, negotiate, and connive your way to your goals?
How do you separate the heroes from the cowards if there are no daunting odds against which to measure them?
How do you distinguish the cautious and prudent from the reckless and foolhardy if all risks come pre-managed?
Screw balance! The best campaigns are the ones that include a wide range of dangers and rewards that the party can face or avoid according to their own judgment. Let the players decide when to hold 'em, when to fold 'em, when to walk away, and when to run. That's a good 75% of the fun.