Full runway length
As the person who originally brought this subject up, I'd like to say that I was not implying it was poor airmanship not to use the full length of the runway. As has been shown above, there are sometimes valid reasons for not using the full length.
That said, not using the full length available is fine if everything goes OK, but it does seem that there are those very rare occasions when the runway behind you at the start of the take off run may have been just enough extra to get out of serious trouble.