In a world full of rip-off artists keeping your mouth shut, promising under penalty that you won't be poaching your employers employees and promising to keep your data carefully and to get rid of it after the job is done is simply good practice.
There are plenty of parties for which signing an NDA makes no sense but if you're a hired gun that will build something or do some work for a fee then signing and NDA is par for the course. Just like any other document you sign you do want to read all of it just to make sure there isn't anything funny in there about you having to eat 10 Kg of brown M&Ms or something like that.
On another note, I would work with contractors on occasion on projects for myself and I would not ask for any NDA preferring instead to rely on my judgement of who to do business with and who not, after all an NDA will need to be enforced if there is ever a problem and that's harder than it looks.
But someone who starts before first contact about how they're not going to sign my NDA (without even finding out if I would require that) is definitely not going to be hired.