I understood mtgx's question to be based on practical grounds, not moral ones. But in answer to your question, I think of espionage as something akin to the military. Perhaps in an ideal world neither would be necessary, but unfortunately the world is not a utopia.
Thus, from a realistic standpoint, spying is sort of like doing market research for a startup. It allows you to make predictive decisions instead of reactive ones.
I agree with international spying. If the other countries don't allow you that information, you must spy to get that information. A scenario of nations having bad or inaccurate information about other countries, their status and their intentions can make the world a more dangerous place.
However domestic spying seems kind of silly. You should just be able to collect the information you need voluntarily through census and and surveys, without resorting to spying. If they refuse to provide the information, get a warrant.
Thus, from a realistic standpoint, spying is sort of like doing market research for a startup. It allows you to make predictive decisions instead of reactive ones.