"It's not bad for your hardware in any reasonable sense. You also need a better anti-virus strategy than that."
Constant rebooting, meaning switching between operating systems many times every day, adds a lot of unnecessary wear. This is true of any mechanical device. Any moving parts will deteriorate with overuse, including those in a hard drive. While there may be no immediate difference, it would potentially, and yes, reasonably, lessen the life-span of the drive. This is why it is suggested to put the computer into sleep or hibernation mode in between sessions that are less than a day or so apart, rather than turning off the computer.
"Well then using Windows as little as possible adds little to your virus resilience...."
As is well known, Windows hosts a great deal of viruses, trojans, adware, spyware, etc. that do not affect the Mac OS. This is one of the primary reasons that people use Macs. Every time I boot into Windows, I am opening my system to these ~115,000 viruses, along with all of the other problems associated with Windows, i.e. crashes, fatal errors, system freezes, driver errors, etc. Regardless of the amount of software used to combat these issues, things quite frequently slip through the cracks. I, along with plenty of other people, have experienced viruses, trojans, and plenty of ads, even with the proper safety precautions in place. These utilities are far from flawless. And, on top of that, virus definitions are not added to the software until the viruses affect at least one computer. The virus always comes before the protection. Therefore, by using Windows as little as possible, all of these threats are lessened.