I have a second passport, but nowadays it's only for Israel.  (Lebanon and Syria won't let you in with Israeli stamps, or with exit stamps from the Egyptian or Jordanian border posts connecting with Israel, even if you don't have the Israeli stamp itself.)  We used to have to use it sometimes to get into the countries allied with different sides in the war in DRC, but that's mostly over now.

With all my Syrian and Saudi and Lebanese and whatever stamps now, I keep expecting the folks in the US to give me a hard time, but they've always actually been rather polite.  I have trouble reconciling my experiences with everything else I hear.

I did get a right sound grilling at Heathrow last year, though.