I know a friend of mine who used other blogging services such as wordpress.com and blogger.com to "mirror" his main site. Personally, I'm not a fan of it. If you're main site is good enough, then visitors will come directly there.

What do you think?

i have seen a lot of people do that but they do it for search engine purposes only and to improve their search engine rankings.

You need to do this to create inbound links to your website. Generally people follow this practice. You are right the basic idea behind every website should be so effective that it catches visitors.....

Regards,
Aarti

Yes i agree

used other blogging services such as wordpress.com and blogger.com to "mirror" his main site

This is trick of SEO or Keyword Position.

I also do it, using same URL in two different blog platform (but i post different content; same topic but i rephrase it to make it a little bit unique.)

having a mirror site will have your site be counted as one by SE, because it have the same content...

To prevent mirror blog, post unique article on each blog. You can do this by rephrasing the article so that it will be unique to other.