Less stretch, less creep, less handshock, more stability, more durability. I'd use Dynaflight '97 or 8125 even if they were a little slower than dacron.
As far as them being harder on the bow....I'm the kind of guy that can tear up an anvil with a Q-tip without trying. I've been shooting the same longbow for over 10 years--it's my go-to bow for tournaments, hunting, playing, etc.--and it's never had a dacron string on it. I pull 66#@30.5", my arrows average just under 10 gpp, and it shows no wear from the string.
Chad