goodroi - 3:36 pm on Feb 8, 2012 (gmt 0)
I have had success combining sites and also splitting up sites. It depends on the situation and your long term goals.
Does the main domain need help? If it needs help can you boost the main domain using other methods like providing free tools, adding more "evergreen" content, improving usability or some other method. Will you lose any backlinks that may become redundant by moving the blog into the main site? How strong are the usage signals for the blog? Do you have the time to deal with moving the blog and dealing with redirects and broken links?
Just remember "If it ain't broke, don't fix it"