If its intended that eventho the friend is invisible but its still occupying space and the sims can detect that, the point of friends being invisible doesnt seem that usefull.
Imaginary means to me its only real to the sim who sees it, it should not exist for other sims. Therefor it should not be able to block or hinder movement.
I think it's kind of cute, like when a five-year-old tells you that you can't sit there because that's where their friend is sitting. It can occasionally get annoying in real life, too, but all you have to do is ask them nicely if they will play with their friend somewhere else.
Alternatively, though I wish they had made it possible to clip through the imaginary friends that you can't see, and then get scolded by the child for being rude.
By the way, I had an elder sim who still had her imaginary friend. It seems like as long as they stay friends they'll age up together. Untill the sim dies anyway, then it turns back into a doll. I'm still waiting to see if she can get it to transform again from beyond the grave.
Nevermind, no luck there.