We are very dry and very hot, so again, depending on the location and the weather conditions you can sucessfully vent into the attic, and as one has stated, I would NOT do this with a kitchen vent. Here you have grease and buildup.
Again, I must quantify, you need good attic ventilation with roof turbines and un clogged soffet vents.
I would classify all the answers to this post very profound and they all fit to their region. Where you have a problem with outside venting and if you have to vent the bathroom, this is only an option.