Well I can tell you why it is a good idea to integrate WP into Joomla. WP is blogging software and Jooomla is a complete CMS. What if you want a site that has a blog, but also has lot's of other functions like an image gallery, guestbook, forum, file repository or even a webshop? Then it really is a good idea to integrate both. You'll have one of the best CMS and the best blogging software in one. To me this seems like a great idea.