No, they shouldn't. They like being independent and are doing well. In the case of Ninja Theory for example, they were struggling and MS said that they would fund and publish their games under them so it made more sense. Insomniac may make more games for Sony than they do anyone else, but it's the fact that they have the freedom to do whatever else that drives them to keep being independent.
Sony, MS and Nintendo should really only build their own talent from now on. It only takes that one person with vision and talent, give them a studio and top tier artists, programmers and you've got something special. Look at Sony Santa Monica, David Jaffe and Cory Barlog had the vision for that studios games. All they needed from there was a good team.