Tints simply reduce the percentages of the color's existing component primaries, for both spot and process colors. Assuming subtractive color (CMYK document), creating shades of spot colors--and of process colors not containing K--would require addition of K (or the complementary primary, etc.) In the case of spot colors, the result wouldn't be a spot color any more. In the case of process colors, you would encounter ambiguities similar to those affecting black generation in RGB-to-CMYK conversion: just as there are myriad ways to generate K, there would also be many ways to darken a color.
In other words, creating tints is a simple and unambiguous matter of decreasing existing percentages. Creating shades would not be a simple matter of increasing percentages, and would have to deal with color-specific ambiguities.
All of what James said. While it technically may be relatively trivial to implement such a feature, the extra work afterwards to cleanup the mess is probably not worth it. Also the more constrained your output work would be (specific profiles and very low ink saturation settings), the less useful it would be in teh first place, as it could never accommodate these automaticalyl and look good....
Thanks for the info. I usually do stuff for graphics and web purposes, so I'm not that familiar with print. I was working on some little karate belt icons for a website and it was getting a little tedious to use recolor artwork and save for web over and over, especially when it messed up and matched up the shades incorrectly. I just wanted to adjust the base color.
I remember learning that C, M, and Y are supposed to make black when mixed but not in practice. I can see why darkening something would be ambiguous. I'll have to see if there's a way to use the edit color group dialog more effectively.
Sure enough, it's fairly easy to define colors to be relative to one another.
You can also think of it this way: Creating tint Swatches is, by design, analagous to the tint screens used in process camera darkrooms; they were physical acetate screens of dots placed under the film before exposure. The screens masked the film, resulting in a pattern of dots being exposed instead of the solid area in the original artwork. The designer would create a black solid in the artwork, and then specify that it be screened to a particular tint and printed in a particular color.
There were no corresponding "shade screens," because it wouldn't make physical sense: You can't expose the film with more coverage than 100%.