Its illegal if you don't own the original, according to copyright laws in some countries including UK if you own the original you are perfectly entitled to make a copy of it and if you ever sell the original then you must destroy the copy within 24hrs.
Wii games can not be copied direct anyway, you need a certain LG dvd-rom and some software to rip the iso from the disc.
I do the same with music cd's in my car, I am one who downloads music but I do actually own over 1000 original cd's, the ones I want in my car I copy, the reason why is if my car gets broken into there is only a few quid of cd-r's to nick rather than £100's worth of real cd's, goes the same for the Wii and any console, why risk the kids damaging the discs when you can just let em play a copy and if it breaks does it matter.