The main benefit of passphrase is for physical attacks.
That's one option to have plausible deniability (if done right) with a sacrificial wallet for an empty mnemonic passphrase, while your main wallet(s) are hidden behind complex strong different mnemonic passphrase(s). Of course, you can't store a physical backup of your mnemonic passphrase any near your wallet or its mnemonic recovery words. Good separation is key here.
I partially agree, 12 words are entropically secure so adding an extra passphrase won't change anything, making the passphrase complex will increase the chance of losing access your bitcoin.
creating a passphrase of 3-5 words is a compromise in case you forget it and it's also an ideal solution for physical attacks, will delay whoever manages to find the wallet seed for a short time until you can send bitcoin from another wallet.
If you forget it, the cost of brute force will not be expensive.