If only one person uses a plugin feature, but that person implements it, its fine. And nobody is forcing anyone to work on anything. Nobody ever knows where capabilities can lead.
No, it isn't. If it adds several thousands of lines to Geany that have to be maintained (and making TM work for both AST and ctags looks to me like a multi-thousand LOC patch which will add lots of complications) then we should really think twice about it. It's not only about writing the patch, it's also about maintaining the code later.
The only request is that people don't resist others making things general enough to allow their different uses.
I'm definitely not trying to prevent anyone doing anything but want to prevent adding too much code that won't be used by many.