I feel that significant others have the right to be present during celebrity photo shoots because it could possibly pose a risk on their relationship. The reason being that the mate that is taking the photo with the celebrity could possibly be put in a compromising position while taking the photo. So that there is no confusion the significant other would see that the whole shoot was professional, hence keeping the relationship in good standing.
When every photo we see of celebrities is photoshopped in perfection and oblivion, it can be easy to buy into the idea that celebrities are somehow of a different species than we are. This is so obviously not the case. Seeing celebs with their partners humanizes them and helps us remember that they, too, are just like us.
Celebrities become such because they have done something worth notice. We "celebrate" them for whatever notoriety they've attained. Though it might be interesting to include them in photo shoots, or have them on hand, it doesn't seem to be something that should mandatory. People want to see the celebrity. They want to view the individual who has gained their interest for whatever reason. Though the significant other may add another dimension to the person we want to see, they aren't the reason the public has found interest in them.
I dont think significant others should be present during celebrity photo shoots. According to me, celebrity photoshoots are meant to capture celebrities and having the presence of their significant others would loose the focus and things wandering around their family or friends . Hence I feel it should be restricted to the celebrity alone.