After seeing all these posts about host migration, I believe I have found out the most logical reason.

Treyarch always believed in "sharing is caring", so they all sat down and thought about how to apply sharing is caring into black ops.

One guy suggested a scorestreak Team Care Package (pretty much emergency airdrop)

Another suggested gun sharing (going up to a teammate and swapping weapons with them)

Finally Vonderharr looked up from his book of bad ideas and said "Lets make everyone be the host at least once per game."

Guy 1 asked "wouldn't that interrupt the game?"

Vonderharr then said "they will love being host. It gives them a sense of pride in their pointless life. I'm Vonderharr and I'm sexy and I know it."

And thus... HOST MIGRATION was born. As well as the image of Vonderharr in a speedo.