I think prompts are like the grit that the pearl of the story forms around? (ZOMG pretentious.) Otherwise there are just too many options. Even the things I don't write to a specific prompt from a fannish source tend to be fairly direct responses to things in canon, so it's as though that was a prompt. And exchanges with deadlines actually make you finish, so for me at least there's a bias towards prompted things in my finished fics compared to started ones.

For me, the thing about prompt tables isn't the prompts in and of themselves as the sheer volume of them, such that they encourage quantity over quality.

I think Ianto's going to die and then Jack will give up his immortality to bring him back to life in a million bad fanfics written immediately afterwards.