The best kind of bridge burning is when the person holding the torch is completely unaware of the conflagration. Well, I guess it's only the best if you are annoyed with the torch bearer and don't care about the bridge.
Otherwise, it's pretty awful to watch. I don't know why it still shocks me when I see people make very short sighted bottom line decisions that will cost them exponentially more time and money in the future. But it does still shock me, because while mistakes are inevitable in life (and work), short sighted mistakes are not. It's bad business.
Workers may be interchangeable in general - I have no fantasies that my employer couldn't replace me with somebody with a similar skill set and experience when they need to - but in the short term, they're not. When there's only one guy who knows how your system works, and you need your system to work next month, you need that guy. Often, you need that guy a lot more than that guy needs you. Programs, businesses, communities live and die around that guy. He's more important than you think.