I've only beta read for friends, but I think it would be a good idea for me to do it more. Maybe I might discover a great writer in he making. I also think it would be a good way to meet writers professionally.
I love it when writing is plausible, smooth and flows evenly. I love the fact that beta reading is an activity that actually requires me to be uptight lol. I love taking something great and tweaking it until it shines and I love losing myself in every tiny detail of a good story.
I haven't yet, but I would if it was a genre I enjoyed reading and also close to finished product. Earlier reading and shaping is more for workshop which is a great idea for new writers where - you got feedback but generally not volumes. Also you critique others and being able to articulate what works is important.
How about you? Why do you beta read?