The biggest thing the Democrats have on McCain is his relative inexperience with economics, which McCain has admitted in the past.  Since the economy is such a big deal these days, that inexperience may bite him.

(Of course, anyone looking at the situation logically would see that neither Clinton nor Obama have any experience with the economy either, but I would count on the media overlooking that fact.)

Adding Romney as a running mate could quell that issue pretty well, since one of Romney's chief strengths is his vast private-sector economic experience.  The guys knows how to turns businesses around, and has done so numerous times.  He's also the person who ran the Olympic committee -- to great success -- so he knows how to manage well.

Now, that's just a strengths/weaknesses thing, and lots of people will come up with that same analysis.

However, I believe there is a strategic reason why McCain may want to pick Romney: to keep him from running against him in 2012.

Romney was not un-popular with the voters.  I believe if Huckabee was not in the picture, things may have turned out a lot differently, and that Romney would be the leading candidate right now.

McCain has 4.8 million votes so far to Romney's 4.1 million.  That's not a big difference, and if many of Huckabee's 2.4 million votes were cast for one or the other, I think that would have closed the gap.

The point is that McCain know that Romney not only has a following of sorts, but that he's probably going to try again one day.  By adding him to the ticket, that keeps Romney from competing with him in 2012.