Dear Will,

First, let me thank you for your E-mail as well as your patronage.  It really makes my job worthwhile to know that people care about what I say and hold my opinions in high regard.

Now, you want to know why you don't need as many land destruction spells as your opponent has land.  Well, the reason is that lands and land destruction spells are simply not played in the same way, and because of this trying to match your opponent 1-for-1 on them is a mistake.  A player can only play one land per turn.  You can play more than one spell per turn (given enough mana), therefore if necessary you can "catch up" to your opponent.  This may seem ill-advised, after all while you're catching up, your opponent has lands an can play spells.  But in reality, it's unavoidable anyway.  Wizards of the Coast refuses to print land destruction cards that cost less than three mana.  Therefore, you can not begin to play land destruction until turn three.  At this point, your opponent will have two or three lands in play.  Maybe even a Llanowar Elves or Birds of Paradise, or at least a Signet.  Best case scenario, you play a LD spell on turns 3, 4, and 5, keeping them at two or three mana.  On turn 6 you start playing two LD spells a turn until the opponent is out of land, at which point you go back to matching him land for LD spell.  Well, what was your opponent doing on turns 3, 4, and 5?  He could've played several 2/2 creatures.  With creatures like Watchwolf, Scab-Clan Mauler, Isamaru, and Jagged Poppet around, by the time you successfully wreck his manabase, he might have enough creatures on the board to win before you can do anything about it.  Not only that, but a good land destruction spell costs three mana.  Three mana on one turn to deny your opponent of one mana each turn for the rest of the game.  It takes three turns for this investment of tempo to swing your way.  And what about creatures and artifacts with mana abilities?  Are you equipped to destroy them too?  The basic message is: over the ocurse of a game, land destruction becomes less and less reliable as they do little to affect the board position.  They simply are not as efficient as simply playing creatures and attacking, which is most likely what your opponent will do.
 

Perhaps I'm painting too bleak a picture.  Of course against a deck where everything costs two mana, Land destruction spells are all but useless.  The point I am trying to make, and tried to make with the other gentleman, is that denying your opponents the ability to play their game doesn't work as well as playing your own game.  A good land destruction deck can stop its opponents from getting off any big spells, but it still must be able to deal with the small ones.  With 12 LD spells, you figure you'll draw about 4 of them against your opponent's 5-8 lands in 12 turns.  That means your opponent will be left with 1-4 lands, while you have 5-8 yourself.  Most powerhouse spells cost at least 4 mana.  That's when the train starts barreling down the track for most decks.  If it takes your opponent 12 turns to get there, they probably never will, or by the time they do, they'll be dead in another turn.  That's a large advantage you've gained.  Are the other 18 cards in your deck prepared to exploit it?
 

Another consideration is spells like Wildfire and Thoughts of Ruin, which can destroy multiple lands at a time.  These tend to be more expensive, and more importantly symmetrical, destroying your land as well.  But if you've held back some land (especially advisable with Thoughts of Ruin) while your opponent has been forced to commit every available land to the board, you may recover quickly while your opponent can not.  One Thoughts of Ruin can take the place of 4 Stone Rain, if you can play it correctly.
 

In short, you should not play as many LD spells as your opponents have land simply because no amount of LD spells will ever completely den your opponent mana.  LD spells are like counterspells or creature removal-- tournament players build blue counter decks knowing full well they won't be able to counter everything.  Removal spells are excellent, but you don't play 20 of them against an opponent who has 20 creatures, now do you?  You simply can not make the whole purpose of your deck to deny your opponent's deck to achieve it's purpose, because then you still need a way to finish them off. 
 

I hope I've explained my views satisfactorily; sometimes things that make sense in my head don't translate into Times New Roman well.  There ave been reams of writing on the subject of tempo and how it works, which is a very similar concept to land destruction and probably akin to this line of thinking, but I won't go any further into it.  I think I've said everything I need to.  But if you ever have any further questions about this or any other Magic-related topic, my inbox is always open and I'll do what I can.

Sincerely,

~BMoor