The same reason as you backup your important files on PC. Your new lens can loose its focus just in the middle of your trip to a tropical island. Your lens can get scratched or maybe humidity problems can occur and you can get some bad blurry pictures. I am even not talking about dust! You will not start cleaning dust or mud from your front lens in the middle of a trip.
|Dirty front lens will cause bad quality blurry photos and increase in auto focus timing.|
Do yourself a favor and do not give up on a spare lens. In my case i am using a Canon EOS 500D (Rebel T1i) and a kit lens which i find pretty good multi purpose lens for day time Canon 18-55mm f/3.5-5.6 IS AF. I prefer using this lens on outdoor activities when lightning is not going to be a problem.
photo source: texasphotoforum.com