Didn't they spare much of London in the same way during the Blitz?
It would seem to me that going out of one's way to destroy an enemies historical landmarks is tantamount to torturing and killing POWs. In that that'll retaliate and do the same. You also engender a lot of hatred in your enemy by decimating such cultural shrines. Even if you're fortunate enough to conquer them it's always important to think not only about achieving victory at the present but the securing peace and stability for the future.
It's hard to imagine, as an America, had the German's and Japanese destroyed the Statue of Liberty in an assault on New York, the Washington monument in DC, or Independence Hall in Philadelphia, etc, that we still wouldn't be considering them our blood enemies today.