We have a person here (a Senior Designer) that dimensions from a theoretical datum in the center of the part and uses the coordinate system. I think this is way wrong. I think that coordinate dimensioning is an easy way out of dimensioning correctly.
Coordinate dimensioning is perfectly legal, meaningful and interpretable. Running all your dimensions from two orthogonal datums makes fabrication and inspection easy, and it makes for a relatively neat, simple drawing.
On the down side, it does not show design intent with absolute clarity.
Machine and sheet metal shops easily can place holes on a flat part all accurately with respect to each other. If you show each separate pattern with composite feature control frames (FCF), they cannot take advantage of it. If you have somebody punching holes manually, a composite FCF might allow someone to make a punching template.
When I started doing mechanical design, I was told by machine shops that coordinate dimensioning was strongly prefered. Today, most shops go straight to CNC, so your dimensioning style is less critical for fabrication. I think coordinate dimensions are easier to inspect.
There is no such thing in ASME Y14.5M as a theoretical datum. Datums are defined by features that can be accessed for fixturing. If the width of the part is defined as a datum, then you can dimension from a centreline defined by the width. Your positional tolerances need to be thought out because fixturing is not absolutely simple.
