Correct. The issue for making restoration-from-mnemonic possible (and figuring out how to handle the UX for it, which a full node with integrated wallet really doesn’t make nice) is Add ability to restore the zcashd wallet from the emergency recovery phrase (mnemonic seed). · Issue #5796 · zcash/zcash · GitHub.
For a brand new wallet, either approach is equivalent, but you’re correct that having the wallet.dat around makes restoration possible with zcashd today.
For an old wallet with pre-UA keys, backing up wallet.dat is a requirement (and one that zcashd-wallet-tool takes care to point out), because the mnemonic can’t be used to recover things that weren’t generated from it. We did implement a hack to bind old Sapling keys to the mnemonic, but there was nothing that could be done for legacy transparent and Sprout which were never derived deterministically.