I am a parent, and I don't understand, not at all - so you're not alone, seashoreCM.
[Parent credentials: one DS, now 24. Graduated Catholic HS w/highest honors, graduated Univ. of Mich with BS in Comp Sci, now living on his own, self-supporting - and still emotionally close to his mother and I.]
As a libertarian-minded person, I generally support the idea that a parent knows what's best for their child; but after reading this thread, I'm not so sure.
While I have no problem with a second grader missing a day or two, I think it's a lot more significant for a HS student to miss an entire week; in 5 days, concepts can be introduced and discussed in depth that will be the foundation for weeks of further instruction.
But what concerns me most are the justifications that some parents give for taking their child out of school. Perhaps I'm just too old-school (no pun intended), as I'm 43; perhaps my parents valued education and good attendance habits more than most. But when I see people taking their children out of school because: it's cheaper, it's easier, s/he's getting older

, we need to form family memories

, or because little Jimmy or Cindy was sad, I just marvel at the human mind's capability for rationalization. In my (tiny little

) mind, it's very clear: education is a necessity, a Disney trip is a luxury.
As I said, I'll support (to the death) a parent's right to decide what's best for their child, but I retain the right to politely disagree in a public forum. And please note: I am NOT trying to say anyone else is a bad parent, nor do I think that I will change anyone's mind on this issue - I intended no flames by this post. I just posted because someone said:
You will understand when you are a parent.
and I felt it important to point out that not all parents have the same opinion on this issue.
IMHO - YMMV