Because I should not have to go out of my way to make a legal activity, LOOK like a legal activity, when it's not even remotely illegal. It's privacy, that's all it is. Lock is to box as encryption is to computer.
We live in a very sad state of affairs that will continue to deteriorate.
Um, you realize that you can't carry locked boxes through customs and then refuse to open them, right? That comparison actually hurts the case for taking encrypted data through customs.
The issue is that refusing to let customs inspect your electronics is illegal. This is the broken thing. Encrypting is just a half-assed way to try to get past this (I say half-assed, because it doesn't work). I don't especially care that encrypting doesn't "look legal". I care that customs has the right to inspect the digital contents of my devices. That is a completely pointless invasion of my privacy. It doesn't reduce crime. It doesn't protect the country I'm entering. It's just an exercise of power.
It's important to remember the destructive nature of Truecrypt's hidden volume function. Unless you use both keys to unlock the volume, opening the outer volume and allowing the OS to load will destroy the hidden volume that has your real files. It's meant to be a last-resort against a "Use this wrench and beat the password out of him" scenario.
I've only used it with a TrueCrypt volume as a file, where it works flawlessly (unless you continue to add files to the public volume). See http://www.truecrypt.org/docs/hidden-volume