Most countries allow you to have multiple passports - it's useful when you have lots of business trips that need visas to 3rd world countries that take 6weeks to process the visa.

I work in mining and in the bad-old-days we used to have to keep multiple passports, to keep SAfrican, Isreali, Chilean etc entry stamps separate.

Now I tend to keep one passports just for the USA - it's easier than explaining to the TSA that visiting a country that doesn't speak English (like Jesus did) doesn't make one a terrorist.

Ironically the only problem I have had is that after being forced to get a brand new passport because the USA demanded RFID - I was questioned for 30mins on my lack of travel history!