I love how, on this forum, any time a thread is started discussing a topic that is new, difficult, or not yet possible to do, everyone tries to tell the OP that it is not necessary, stupid, or unreasonable. What a joke.
The reason is very clear. He has a bluray movie disc (that he presumably does not also have on DVD) and he wants a copy of the movie to watch on his iphone. What more do you need to know? He didn't ask if it was a good idea, and didn't mention once that he's doing it because he thinks he'll achieve improved video or audio clarity or anything of the sort.
The reason why anyone would need this are simple. For high quality TVs, with large screens (60"+), the difference between DVD and bluray is night and day. Therefore people buy blurays instead of DVDs, since the primary use for this optical media is for viewing on their hometheater, where they notice a big difference between the two formats. If these people then want to watch the same movie on their iphone, they have a need to rip the bluray disc. Obviously, in this case, they have no need to rip it in full 1080p resolution, but they still need to rip it. Saying that this is not necessary is just ignorant.