For me it's not whether it's useful, it's whether the tradeoffs are worth it. I assume GDScript has some unique features that make it especially useful in Godot development, but the disadvantage is that the skills aren't transferrable to any other platform or environment. At least C# and C++ are applicable outside of the engines that use them.
I get what you're saying, but a good chunk of transferable skills are not language-specific. Programming in a video game environment is going to impress some knowledge upon you that is generally useful, regardless of the language you use.
Because as is C# is very noticeably a second class citizen. Something like Godot 4.0/4.1 missing C# support for android/ios/web shows that perfectly imo.