Islam is a religion of extreme externals. Nearly every move of the believer is bounded by religious traditions and commands, from the complicated motions and verbage required in prayer to the correct way to handle your personal sanitary needs or blow your nose. It is the ultimate religion of "works" and "rules".
Wrapped up closely within this structure is the crucial importance of Mecca and Medina and the religious shrines and traditions associated with them and required by the Hajj.
Rather than go into detail, I refer you to this book (in pdf) by Edward Sell, a British scholar who did extensive research on this religion. See The Faith Of Islam. For further reading, also see other books available here
When reading Sell's work, go to pg 331-347 (pdf pg 346-362) for intimate details on the Hajj. Pay particular attention to Sell's conclusions on pg 347 (pdf pg 362) and you will see the same conclusion I asserted.
I am not of the resident "Nuke Mecca!" clan, which shows up ranting for this destruction on this forum every time a car bomb goes off in Baghdad. Contrary to their view, the destruction of Mecca and it's environs should be contempated only as a last resort, the Armageddon Option - akin to America being in Hitler's Bunker in May, 1945 with no other way out.
As I have stated (as do you), the ensuing Jihad over this destruction would be terrible to behold. But it may in the end be the only thing which could ultimately bring down Islam into the ashes of the rest of the world if the only alternative is the total destruction of the West with only Islam left standing.