DMD Compiler 'switches'

[ric maicle via Digitalmars-d-learn]

I'm relearning D. I'm using the reference compiler (DMD) and I am a bit 
confused with how the compiler 'switches' are supposed to be used.

I find some 'switches' that require an equal (=) symbol when a value is 
required to be passed in.

-boundscheck=[on|safeonly|off]
-color[=on|off]
-conf=path

-debug
-debug=level
-debug=ident

But there are also 'switches' requiring a value that do not require/use 
the equal (=) symbol.

-Dddocdir
-Dffilename

-odobjdir
-offilename

I'm wondering if this small irregularity should be made consistent or 
maybe I misunderstood something.