I have been researching this and I still have some questions regarding this. Some of the responses have said that it is a dangerous thing to do because you could be granting greater permissions to profiles by doing this.
In my situation, I would be moving them from a Production environmnet which should only have more tight security than the sandbox I would be deploying to - so I wouldn't think I would have to worry about granting rights to anything greater than it already is in sandbox.
Another question I have is : If I am deploying a profile from Prod which already exists in both environments to a sandbox with objects not yet in Prod will the deployment take those into account and keep them in the profile during the deployment ? In other words, in the end I would have a complete profile.
If anybody has any other advice\warnings with this or would like to share their experiences I would be very grateful.
Thank you very much.