Some people like non-confrontational ways of saying "No.".

Sometimes you really do need to think things over before making a decision. "No" closes doors before you've had a chance to consider.

It prevents weasley salesdroids from seizing an opportunity to take you to the cleaners.

Please read what I wrote again.

How does "I'll get back to you after considering" let "weasley salesdroids" take advantage, as long as you hold firm? If you're not the type of person to hold firm after saying that, you won't hold firm after saying "No" either.

Because people don't hold firm. You're just inviting them to pull out another one of their lines.

I think you either fail to read comments in their entirety or just focus on one thing. So I'll say it again "Someone who can't hold firm won't hold firm. It doesn't matter if they 'No', or 'I need to think about it'".