I've never seen anyone try to take 15-point Immunity as a "Can get away with murder" advantage, and I'd certainly require an implausibly good justification if anyone tried that in a game I was running. The diplomat John mentioned from the THS game was effectively spending the 20 points to be the Serious Accredited Diplomat of the party, making him the point man in dealings with local authorities and so forth.
One could argue that the base cost for Diplomatic Immunity (before the extra 5 points for Diplomatic Pouch Privileges) in modern-day games might often be less than the 15 point version, depending on your employer's attitudes and the state of their relations with the host country. (And note that, by the Vienna Convention, diplomatic immunity only applies in the country where you're an accredited diplomat.) Your employing nation will at minimum be very annoyed if you embarrass them, and is actually free to waive immunity and hand you over to local justice if they're annoyed or embarrassed enough. Or they may prosecute you under their own laws. Or they may recall you, or the host nation may expel you, and you may find that you've just terminated your career, or at least screwed up your promotion prospects.
Going by
the Wikipedia entry, yes, a few diplomats have literally got away with murder -- but others have suffered actual consequences. I think I've even heard of cases where diplomats who'd behaved like idiots suffered
worse consequences after recall than they would have under the host nation's justice system; murderous dictators can be very annoyed at losing face. So perhaps a diplomat with few special connections, from a law-abiding country that values its squeaky-clean image, should be paying just (10+5) or even (5+5) points for Diplomatic Immunity. Heck, some may not even have useful Diplomatic Pouch Privileges.
I've not seen other forms of Legal Immunity come up at all in games, which actually makes me think I've not tried hard enough. Hmm...