I think that since Square-Enix already has a number of development divisions it's not necessary. Compare FFXII and FFXIII and you can see from just how different the two games are that Square-Enix is already a company with a staff of diverse talents. I don't see why the company would need to look to external developers to contribute to their titles when they already have such a large, dedicated staff capable of handling numerous titles at one time.
Having said that, Square-Enix have already been working with a number of external developers for some of their smaller projects. Matrix Software, for example, have worked with S-E on the Nintendo DS versions of FFs III and IV as well as The 4 Heroes of Light. In my experience, FFIV DS is a very high quality game. I don't know exactly what Matrix Software contributed, but certainly they did not have a detrimental impact on the game.
So I would say that for the major, blockbuster titles, S-E already has the resources it needs without having to outsource. However, given that so much is put into the biggest development teams, presumably leaving little else for other projects, it is understandable that for smaller games they would look to other developers for assistance.
Now that S-E is such a big company with many assets in the USA and Europe, I wonder if they will use these for their traditionally Japanese-developed projects. The conservative structure of Japanese business makes me doubt it.