Why rebuild make oneatatime compulsory?

[davidl]

From my point of view , with the incapable of DMD header generating the  
one at a time compiling option can never boost up the compiling speed. So  
with the give situation why still rebuild make compiling oneatatime as  
default? It's even annoying of the current compulsory behavior. It tries  
to compile each file seperately not feed to the compiler in 1 command line.