I'd like to use the symmetry constraint for more than just vertices; I'd like to use lines as well.
The two vertices in the attached image are not the same length, therefore I cannot choose the vertices I've pointed out in red. But if I could choose the LINES themselves, I could use the vertical symmetry I'm looking for that would replace the 109 dimension.
If you dropped a center point on each yellow line, you could then use symmetry and remove the dimension. Obviously that's just a workaround but until this idea winds it's way through the system, at least it helps!