If each invidual site is not geared towards a subject/phrase(meaning very targeted) I would combine them. But if each site is very targeted I think they are worth more standing alone. And this is dependent on how you go about getting your customers. If it is through other kinds of advertising it may be different. But for the search engines, I’ve found the more targeted the better.
MSN is a wildcard. Not now, but within the next year, we may see actual competition with Google. And if their beta search is an example of what we may see in the future, you will want targeted sites on one subject. Now, if they change, and start counting links from every blog or message board in the world, it will have the opposite effect.