Would there ever be a practical reason to a get search warrant for a helicopter thermo fly over?

There's nothing actually wrong with them using thermal equipment. They just can't use it to get a search warrant, meaning they use it to find people to follow then wait for them to do something wrong. They would also be able to use it in court, and they wouldn't need to tell you that they were getting a search warrant. i.e. you wouldn't know they were attempting to find reason to arrest you.