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.
I don't know if this helps, but you can:
export the private keys from both wallets
Better use
dumpwallet on wallet1, then
importwallet the dump file to wallet2.
That's doable within a minute if you're fast, minus the rescan.