But to try and play down the usefulness of expensive lenses is just denial
I’m not denying they are wonderful things, but the message to the beginner is do not think for a minute that you NEED one to get good pictures.
To answer the OP’s question directly, it’s absolutely NOT worth it unless you frequently come up against a situation where your lens does not perform and an expensive one would. AND you think it’s worth spending the extra.
To compare with MTBs, you do pretty much need to spend over say £3-400 to have a fun time doing what we’d consider proper mtbing. You definitely do not NEED a Yeti ASR Carbon to have fun unless you know you’re going to benefit from it, or have deep enough pockets not to care.