1) NaNo is a tool, not an end in and of itself.
2) If achieving wordcount is the real goal--and it is, in so much that that is the only way to "win" NaNo, then count every word. If you delete more than ten words as you go along, you are (in my humble opinion) doing it wrong. You don't delete. Use the "strikethrough" option if you don't want those words in the story, because you wrote them in the month of November and they count.
3) There's a line of patter about the purposes of NaNo, and you might believe it. Or not. So here's MY best reason to do it. We all know that novels are not (unless they are Middle Grade or early YA) 50,000 words long. For my purposes, NaNo's great function is that it cheerleads you past that 25-30,000 word mark, where a lot of novels stall and die.
And those are my NaNo thoughts.