Advantages of Monorepos (2015)
It's very simple: with a monorepo you always have access to everything you need, together with a ton of stuff you don't.  Whether or not this is advantageous boils down to whether the cost of not having access to something you need is greater than the cost of having access to a bunch of stuff you don't.  As long as your system is reasonably efficient at letting you select small subsets of everything you could potentially have access to, the cost of having access to a bunch of stuff you don't need is essentially zero.  Perforce is good at that.  Git isn't.  So people who use Perforce tend to think that monorepos are good and people who use git don't.  And they're both right.