You'd lose HD wallet feature, which force you to backup wallet.dat file regularly. OP apparently trying to merge multiple wallet.dat without losing any features or advantage each wallet.dat have.
He will only need to backup once after he imported the keys or dumpwallet file.
After that, the 'new backup' can still generate the new/future addresses/keys as long as the hdseed is the same.
Old backups however wont contain the imported prv keys.
Thanks for the correction, i totally forget you only can create HD wallet with newer version of Bitcoin Core and Bitcoin Core always generate address from HD derivation path (even if you import private key)