Not quite what I'm getting here to be honest :) It is not a major issue at all because right inside the tri-planar shader there is the same functionality on offer - sure, it's a bit surprising it doesn't work in the diffuse channel but we can easily get the same result, we just need to go inside the tri-planar shader.
edit: The same issue appears when you for example place a tri-planar shader inside a color mix to drive glossiness maps or stuff like that. Whatever is on top of the tri-planar shader simply doesn't want to take mixing into the account (like the mix strength slider in the color mix shader). Again, I personally wouldn't consider this a top priority issue because the workaround is to go inside the tri-planar shader and use the mix strength in there. Still, it would be really nice if this issue isn't forgotten.