just a feature that is currently missing. Why wasn't it added before now?
The Suggestions page provides insights into the two most likely answers to the above, perfectly fair, question: a) other features are considered more important and have been done first, and/or b) there is insufficient consensus that is a "feature" which should be added at all, let alone at higher priority than other things.
I personally like the idea of fatter or thinner area border lines even though there are plenty of people who think that is sloppy cartography. It's not a 100% guarantee that everyone will agree just thickening lines is a good thing. In fact, although a nine-word note is enough, "Please add area border line width control to style," most people who comment on such things with suggestions don't write that but instead discuss the more sophisticated case of asymmetric border line styles.
That's what people really want. They don't want an area borderline that is just fatter, the way you can often get away with in the case of a line. They want a border line that is fatter or different color only on one side, such as the inside, so that the cartographic indication of the actual size of the area and what the area covers is not altered. Or, they want that for better cartographic effects, such as gradient inward from the area border, or, in the case of some cartography such as the rendition of islands in seas, a gradient blend outward from the area border.
Even given the adjustment to community driven feedback that the Cutting Edge process allows. I think it important to try to close in as close as possible to what likely be a final approach. That also includes desires such as customization of styles, asymmetric line styles, sufficient facilities to be able to do what people can accomplish with things like SVG, and the ability to do all that with high speed, not slow as is the usual result in other GIS or server settings where very extensible, but highly bureaucratized and gruesomely inefficient, approaches to stylings give wide cartographic variety at the cost of reduced performance. Manifold has to deliver full variety with high speed as well.
Where all that fits in depends on where it makes sense to fit it in, for example, holding off until better innards are available to achieve better and faster rendition on screen and in printed output. As it turns out, as I mentioned in other posts, given the massive progress on innards accomplished these last few months, as Manifold switched gears a bit since the last series of builds all that (custom styles, better cartography, etc.) is now in progress. So far, it seems it will be a good fit to what the community wants in terms of the suggestions received.
Once the cutting edge builds come out we'll see: there will a chance for feedback from a broader audience, to make any adjustments required.
As always, it is never too late to send in Suggestions if anybody has any specific desires in mind.