I recollected the case with the Trust Wallet browser extension, where  weak PRNG led to a serious vulnerability, namely, hackers were able to exploit this flaw and steal users' funds due to poor SEED generation.

Since no one can predict which wallet may expose the flaw in SEED generation, adding a custom SEED extension with an extra phrase would provide the additional layer of security for your wallet.

Personally, I always use a passphrases to extend  SEEDs on all my wallets and strongly recommend everyone do the same.