As per your question about why Thompson would want Romney: Money, and managerial competence. As part of a ticket, Romney could presumably continue to finance a large part of the campaign. Meanwhile, one of the biggest knocks on Thompson, as per National Review, is his lack of any executive or serious managerial experience. That is Romney’s biggest selling point. Also, Romney helps Thompson excape the “regional candidate” image, and Romney’s base of Mormons nationwide can be a large organizational help. Finally, the one thing THompson needs right now more than anything is attention and a sense of momentum. It seems to be South Carolina or Die for him, and to win South Carolina, he must shake up the race. Nothing would shake up the race more than an unprecedented move such as naming a running mate so early in the process. What Thompson needs is drama, and this would create it in abundance. Plus, of course, if he takes even three-quarters of Romney’s voters in South Carolina, those extra voters alone would help vault him far closer to the top.
Also, your analogy to the Reagan/Ford thing is not apt. First of all, all of that talk started AFTER Reagan was the sure nominee, as part of a last-minute idea at the convention to unify the party — NOT to win primary votes. Second, it was a different model: Ford specifically was demanding a co-presidency, with himself (NOT Reagan) having primacy over foreign affairs. The Thompson thing follows more along the lines of a business model, with Thompson clearly in charge of policy decisions but Romney in charge of implementation. In short, Romney would clearly be subordinate; Ford, as a former president, wanted to be co-equal.