Like many other things, the decision to pull a child out of school or not is a personal decision and depends on both the child and the family. My only recommendation is not to write a letter, but to speak personally to the teachers and school administrators.
With respect to calendar changes. Our district changed the calendar for 2008/09 last spring!! Instead of missing the first 3 1/2 days of school, they ended up missing the first 8 1/2 days of school this year while we were on the EB Repo!
For our two youngest, the principal gave us their math books ahead of time and they had the suggested sections completed before leaving on the cruise. We also had the kids read most everyday while on the cruise.
After getting back, we immediately spoke with all of the teachers (we didn't even know who they were before we left!). They were all very understanding and the kids received excused absences on their assignments. A month and half later, they are all receiving the highest grades possible and at Parent-teacher conferences this past week, we heard nothing but praise for how the kids are performing and behaving.
Like many others, our kids perform well in school so missing a few days isn't that big a deal. However, if our kids were struggling, I think we'd think twice before pulling them out.
One other difference is that we're very involved parents in our schools. The teachers and administrators know us, especially my DW, and know that we will do what it takes to make sure our kids are getting the most out of school.
For our next cruise, we don't plan to pull the kids out of school but will cruise in early June. Yes, it's more expensive but our oldest will be starting high school after our next cruise so don't want to pull him out anymore.