Because it may be really *expensive*.
Fundamentally, if ever a system is taken out of service, whatever redundancy mechanisms may be
in place may happen to fail then. Thus it is wise to reduce the time for which a machine is
out of service. ksplice can, for a large proportion of smaller patches, reduce this period to
a fraction of a second, with no loss of non-persistent state.
And that seems worthwhile to me.