The first things I always think of when I see this is: 1. The "Other site" most likely took a very large investment 2. The "Other site" has been online longer then the clone or mirror 3. What makes the new clone better? Why would you waste your time trying to copy someone else's business model instead of creating your own that may be similar, but is unique in some important way. Just some tips for people who want to copy another site. I wouldn't waste your time, you mine as well just buy a good template, or better yet, have your own unique design created.
Its not people who want the clones, but webmasters thinking they can get the same traffic/revenue by copying. This is a dumb idea IMO, just create useful sites, niche sites and good ideas that haven’t been created yet!
Or often they are simply too lazy to create their own, thinking they can simply copy what they already know works.
You guys are missing the point. Clones are useful because the owners may want to target a niche. For example, a YouTube clone is useful for a site that wants to offer a video service for paintball teams who want to upload and watch each others videos.
People want to copycat for many reasons, it can be quicker, and is hoped that it can be as sucessful as the original by the copycat. As in the youtube example, there a tons of copycats out there, some who may make a decent buck. If an idea is new and isn't littered with copycats, then it's probably a decent idea to build a similar site. Look at facebook for example!