the elimination of an often used shortcut (CTRL+ALT+Backspace, on the default install)
If the X server needs to be killed often, that is the real problem, not disabling c-a-b. The average end-user shouldn't have to be killing the X server. For the technical-minded, there are trivial ways to kill X if need be: switching to a vterm and remedying the situation, using the magic SysRq combos, and setting DontZap to false so c-a-b does work.
"Don't drop acid, take it pass-fail!" -- Bryan Michael Wendt