There is nothing wrong with multiple domains, though I have no idea why people would want to do it.

There is a problem with multiple copies of the same site. If multiple domain names "point" to the same site, Google sees these as multiple copies of the same site (ie the URL's in the address bar are all different) --> asking for problems. The only wa a rond is to to do a 301 redirect, so all the multiple domains point to the one URL (ie the URL's in the address bar changes, rediecting to one)

CBP