> To my knowledge, there are no display devices in common use that natively display vector images.
> So vector images need to be rasterized into bitmaps at some point anyway, with a somewhat unpredictable loss in quality as a result,
> unless the resolution of the target bitmap is very high.
I think you missed the point.
If the diagram originally had a line at some angle other than 0 or 90 degrees,
this line can be optimally rendered in a bitmap of the viewing size (e.g., by Bresenham's algorithm).
If, however, it was previously rasterized into a bitmap of some other size,
and now needs to be converted into a final viewing size, there will be a significant loss of clarity.
If the final viewing size is small, the effect of being seen as a "line" can be totally lost.