I don't "need" it per se. I am using Plex to stream to 4-5 clients of varying capabilities simultaneously and streaming optimization is just part of some library overhauls I am doing to try and decrease buffering on the client side. The Plex forums mention that in addition to other factors (using .mp4's, limit x.264 parameters to high 4.0, including a 2 channel AAC soundtrack as the first sound track in the container, etc.) there would be better compatibility with more devices reducing the need for transcoding, which in turn reduces server load hopefully reducing instances of buffering when Lots of streams are in-progress. Since I am already re-muxing my existing library into a uniform format I am doing streaming optimization as part of the process. Unfortunately I am finding that the script I am using, while doing a great job overall, has some issues pulling meta-data which results in about 25% of my movies needing to be ran thru meta-x after they've been re-muxed. Consequently I now need to wait on Meta-x to re-optimize the file instead of just being able to write the meta-data.
It's not really a big deal in 1's and 2's, but batching thru my 2500 movies while redoing my library the 2-3 minutes each tend to add up.