what is wrong with plot by colour.
Well, I suppose in the big picture there is not really anything inherently "wrong" with plotting by color, BUT consider this...
After staring at a computer screen for 10 hours a day, some colors simply make my eyes hurt, but this in itself is not a good reason, however, considering that you can set a lineweight to the layer, so as long as you are drawing in the correct layer everything works fine. Also, as a rule caddies are visually cued by the color of objects, thereby allowing the user to know with a simple glance what layer or type of object something is. It has also been my experience that some plotters (specifically HP) do not plot ACI colors greater than 12 correctly when using CTB, while other plotters will plot fine. The lineweight is changed by the plotters for some inexplicable reason.
You will also note that when you are sent a drawing by someone else, if they use a CTB for plotting, they almost NEVER send you the CTB file, and you are left to fend for yourself, on the otherhand, the lineweight solution resolves that because lineweight is stored in the drawing, not a seperate file.