Why is plymouth so important that cannot be removed on ubuntu desktop 11.04?
It has priority required and
cryptsetup depend on it.
I do not have really any need to remove it, but a friend asked, so I am just curious.
As htorque noted in the comments, if you remove the plymouth-theme-* packages you will not have a graphical boot. Ubuntu Developer, Upstart author, former Canonical employee, and current Googler Scott James Remnant wrote in a relevant bug report that "without them Plymouth merely regulates access to the system console in cases of filesystem decryption and error."
Ubuntu and Debain developer Steve Langasek adds in a message to the Debian development mailing list: