I have done web and graphics design for more than 11 years and ministry for more than 16. I am currently a church planter.

Here are my reasons:
1. Keeping your blog offsite with links back to your church site help increase your Google rankings.
2. An offsite blog can help connect with non-church people who will not go to a church website.
3. Keeing Blogs offsite helps streamline your church site and your blog site.
4. Offsite blogs can have a unique domain that allows people to quickly get to the content they want and not filter through other stuff they don’t want.
5. Using RSS/Atam feeds your offsite blog can become a second, and sometimes more convenient, portal to your church information.
6. Using RSS/Atom feeds, your blog can be integrated back into your church site as I have done on several pages. I just link titles, but using Joomla you can actually integrate the entire blog into the church site while still hosting it offsite. Then your blog gets Google ranked in both places and not just one.
7. Using offsite blog hosting, a church can also create minitry specific sites that target a group and allows them to get their specific info wihtout extra stuff they don’t want.
8. Hosting blogs, for the pastor or other ministries, offsite reduces the need for in-house web management and the need to hire a professional designer.
9. I am not trying to sell anyone anything and my advice wont cost you a dime.

Finally, here are my sites you can check out.
Our church http://www.ortingreunion.org
My blog http://www.morethancake.org
Our MOPS blog http://www.ortingMOPS.org
The blog for our second campus http://www.ReunionWest.org.

Just my 2 cents :-)

My Blog: http://www.MoreThanCake.Org
My Church: http://www.OrtingReunion.Org

Here are a couple reasons why I chose not to.
1. Hosting offiste with links back to the church site helps increase my Google rankings for both my blog and my church site.
2. By hosting my blog on a seperate site with a unique domain, it makes it easier for folks to find the content they want and not force them to look at content they don’t want.
3. With RSS syndication, I am able to turn my blogspot into a second hom page for my church site. People can get all the essential calendar details from my blog and link to the church site for more details they want.
4. With RSS I am able to syndicate my blog to the church website and still communicate effectively while reducing clutter.
5. I am not selling something like the guy above by making any plugs “shameless” or otherweise.
6. Using sites like BlogSpot, I am able to create dedicated spaces for specific ministries to showcase their stuff and I, as the web designer, don’t have to do anything to keep them up do date. This is not possible on a fully integrated all in one site. See our starting pages for our MOPS ministry at http://www.OrtingMOPS.Org.

In summary, offiste hosting for blogs and other ministry specific pages allows for
1. Streamlined content
2. Increased Google rankings
3. Itegration through RSS/ATOM.
4. No web professionals need to be hired.
5. Blog software stays updated with the latest technology and no one in the church has to do the work and you don’t have to pay a web guy.

That is my 2 cents.