I would use this feature but I'm not fond of putting a preference in the main UI like that. It would probably fit better in a plugin preferences tab.
More importantly though I think it would be hard to implement cleanly/easily as the core of Geany would have to try and let plugins handle going to definition and if none did then it would have to fall back to the original behaviour. This sort of sound like the "providers" API that was previously discussed for plugins [on the mailing list](https://www.mail-archive.com/devel@lists.geany.org/msg02841.html) (TLDR; allowing plugins to hook in to builtin features like this).