You probably won't spend time and money for your media center in your living room or your gaming rig. But let's say, for example, you are a company developing bleeding edge somethings you need to keep a trade secret for the time being. If it's big enough a deal, you'd probably be willing to spend money to make your system is secure from certain agencies that consider it "terrorism" that you're developing something their nation's companies do not have. If you can't get the source code, throwing an infinite amount of money and security experts on it still would not help a bit.
But okay, let's say it is only bugs they would exploit. Great! Luckily, bugs can be fixed, software can be patched. Oh, you have an exploitable bug in a firmware blob/driver blob/proprietary software? Sorry, you're out of luck.