And yet another reason for two drives is performance. Queuing for the drive can be a major performance limiter, that's made worse if there is only one drive and everything is trying to access it at the same time. Two drives mean two queues, thus half the queuing time. In reality though putting Windows on a separate drive means that Windows is never delayed by a read/write for user data - and since Windows is running everything, that matters.Another reason for two drives is if you have problems with Windows often the best solution is a fresh install and you would not want to wipe all the other stuff on the machine when you do that.
I always keep a back up drive of my data anyway, USB drives are not expensive and if only for peace of mind it is nice to have a copy of everything saved in case something does go wrong.