As described in the wiki when an extension registers a new tag, signatures within the tag won't be expanded because preSaveTransform skips all registered tags.
I am currently working on an extension where it would be very useful to expand signatures within the tag of my extension.
And yes, I am aware of the {{#tag:tagname| workaround, however for my use case I need a tag instead of a parser function.
Another use case would be to allow the expansion of {{subst:}} within tag attributes, which would also be very handy.