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Why not just have the one website?
I also see some where the website is just pointed to the blogspot, so this is just an exact duplicate. Isn't it bad to duplicate your entire website?
Wordpress does give you much better management but you need to know some programming to be able to do it.
Outsourced site - blog managed by non-techie in house
Simply not aware that products such as Wordpress exist.
I am sure that others can think of a lot more scenarios.
Remember that a lot of sites are built by non specialists who make decisions that will seem strange to the more technically minded.
1. blogger doesn't need a mysql database
2. Blogger blogs get ranked better than blogs on other domains
3. The server load is on googles shoulders not yours
4. Better exppose because people can easily find your blog if they do a search on blogger.
5. Some people started with a blogger blog and they don't want to have to try to migrate their reader to a new location for the same blog in fear of losing some.
These are just a few of the reasons.