Of Chestnuts, Elms, Ash, Sycamores, and Ginkos
At one time the entire eastern half of the United States was blanketed in Chestnut trees.
If you visit a building built before 1900 you see the rich warm color of chestnut wood everywhere. Thanks to the Chestnut blight of 1904 practically the entire population of 4 BILLION trees was wiped out.
The middle of the last century was a golden or should I say green age for Midwestern towns. Towering elm trees lined every street, arching over and creating arboreal tunnels. 
Until Dutch elm disease wiped them out.
The latest scourge of the Midwest is the Asian Emerald Ash Borer, a stupid little bug that burrows under the bark of mature ash trees, completely girdling the tree and killing it within a single season. In the county I live in there are reported to be over 4 million dead trees already.
Recent trips to Europe have given me cause for concern. Everywhere I travel, but especially the UK and Germany, there are beautiful Sycamores. 
Their huge trunks and evident heartiness lend character to stately homes and public parks. A Sycamare plague would devastate the town and country alike.
Which brings us to the Ginko tree. 
There are no known pests that attack this prehistoric tree. Wherever I see these trees it is gratifying to see their bright green leaves and somewhat primitive structures completely unblemished. Try a walk down East 80th street in Manhattan to see an urban forest of Ginkos.
I think you can predict where I am going with this. Monocultures are very risky. It is nice to have uniform trees lining our streets. The effect when they all leaf out together is stunning. But, the risk is that after decades of investment in maintaining these beautiful trees everything can be wiped out in a period of months.
The cost savings and advantages of using the same operating system on all computers has to be weighed against the risk of global devastation of those machines brought on by the next virus or worm.
Should we cut down Ash trees because they are vulnerable? No. But should we be planting Ash trees? No! We should diversify!
Are you expecting me to compare the Mac to the Ginko tree? Maybe I could. Neither one has pests that prey on it.