"I agree with you that a week in a year of otherwise good attendance is unlikely to have a lasting impact on a child's overall attainment"
Well there you go then. You agree it's not a problem too
"If I can do it once a year why not twice? If they didn't miss much during this week then it won't matter if they miss next Tuesday. The principle is important."
Do you like a glass of wine, fivecandles? If so, how do you know you might not want a whole bottle next time or even 3 or 4. Perhaps you are on the road to alcoholism?
Why would one holiday in term time lead to having time off here, there and everywhere?
"Teachers and schools need to say it's not acceptable to miss school without very good reason"
But who should decide whether it's a good reason? The school, the parents? Surely it should have input from both sides with a variety of things being taken into account.
"because otherwise they're saying the education they provide isn't that valuable"
I don't think that's so. I think it would be a recognition that there is more to life than school and that individuals count and their circumstances should be considered.