The point of relighting strategies is to give plugins some flexibility over how certain calculations need to be made.  I guess NEVER could be dropped, but there are certainly valid reasons why a plugin might want to choose IMMEDIATE/DEFERRED/STATIC options.  IMMEDIATE is only a little faster than plain old Bukkit (at least where block lighting properties have changed), and will certainly result in server lag for very large changes.  STATIC could be very useful to plugins who want to maintain a fixed lighting level over a region in the world, e.g. a game board.  DEFERRED is the best option for really huge changes where "natural" lighting is desired.