Ok we know that Magento deactivates the FPC for SSL pages. Also how to fix it and that we anyway should use an alternative. ;)
But for what reasons is it deactivated? The only one which I can think of that SSL is per default (almost?) only activated for the checkout which might contain sensible data. To prevent from a possible flaw these sensible data would not be cached at all. But in this case it would be a lot better to just deactivate FPC for the Checkout controllers and not for all SSL sites, especially as Google now also cares about SSL and you might activate it for all pages.
Are there any other and even better reasons?