Actually both Sitka Spruce and poplar will generally run lighter than POC. There may be others--I'm no arrowsmith, and there's lots I don't know about wood.
POC will usually break easier than some hardwoods, but quality arrows aren't easy to break--especially if you keep them on target. I've found that if you miss, no arrow material has dibs on which one is easier to find (without a metal detector at least, where aluminum has a slight edge).
I prefer POC for the reasons Ferret described. Of all the woods I've tried--maple, ash, sitka spruce, douglas fir, logepole pine/chundoo, laminated shafts, and probably a few more I don't recall off-hand--POC has had the best over-all qualities for arrows. One of the unique qualities of POC is most of it's moisture is not water, but oil. The sad part is good POC is getting harder to find.
My experience with hardwoods is they are a royal pain in the butt to straighten and keep straight. The one time I tried maple shafts, several of them broke on the first shot while bare-shafting into soft foam. Probably bad shafts, but enough to steer me away from them.
FWIW, I have no problem getting arrows in the 600-650 grain range with 125 grain points, and I don't do anything extra to add weight.
Back to the topic though.......no doubt killing shots can be made with lighter set-ups, and as long as it's legal I won't condemn someone for using what works best for them. I just don't feel comfortable pushing the limits myself, and speaking for myself and my personal experience in shooting and watching fast, light arrows being shot, there is no real advantage--at least within what I consider reasonable hunting distances. I've probably seen (and made) as many shots go high as low when they were mis-judged. I guess the mental aspect could be accounted for--if you think you will shoot better with a certain set-up you generally will, even if there are no physical advantages.
When you get right down to it, nothing will replace time behind the bow as far as being able to put the arrow where it needs to go.
Again, I hope that nobody reads this and gets the idea that they can magically add 10-20 yds to their hunting range just because they swapped over to a faster set-up; or think "well, I'm not real sure, but since I'm getting X-fps I'll give it a try".
Chad