Generally there is value in carrying a second (or third) passport to enter a country where diplomatic relations are better or where visa requirements are less stringent. When I went to Turkey last year my friend entered on her NZ passport as there was no fee for the visa for her while I was stuck paying $20 for mine. And you are supposed to enter a country where you hold a passport using that country's passport. Other than that, not too big a deal that I can figure out.

As for "sneaking" a second passport that you aren't supposed to have, I wouldn't go there. I don't see the upside as being good enough there for the risk.