I would love to pay £15 for Vegas Pro 14 Edit via the Humble Bundle if I know it will work for life. However, I tend to upgrade my hardware fairly often and perform regular fresh installs of Windows. What is the activation system like? If the software is going to treat me worse than a pirate, then it's just a waste of money and I will just have to stick with free software instead.
Seems like Humble Bundle is legit .. Once Magix activates, that should be it for that install. New installs will need to be activated again. That should be ok too but I don't see how anyone can guarantee the future 100%. My opinion is you have little to worry about at the price you'd be paying.