You need not merge any wallet.dat or anything. Just on the PC you will place this wallet.dat, you will have to download the next parts of the Blockchain.
Bitcoin is NOT based on wallet.dat files but addresses. That address has those BTC's and Transactions.
While the advice is sound, the details are mis-leading.
With default settings, Bitcoin-Qt pre-generates the next 100 addresses (and their private keys) that it will use and stores them hidden from you in the wallet.dat file.
Each time you use a new address, Bitcoin-Qt grabs the next one from this queue, and generates a new one to add to the queue so there are still 100 unused addresses pre-generated.
This means that when you backed up your wallet.dat to the USB, you didn't just back up the addresses you are currently using, but the next 100 address you will use as well.
When you restore the wallet, it will compare it's list of addresses (including the 100 that were previously unused) to all the addresses that are in use in the blockchain. If the wallet finds any transactions in the blockchain sending to or from any of the addresses in the backup, it will recognize those addresses as "in use" and will move them from being in the queue to the list of used addresses.
Note that your wallet will create a new "in use" address every time you create a transaction to send bitcoins and will send the change there.
This means that if, after backing up, the sum of the receiving addresses you create plus the number of transactions you create is greater than 100 then your bitcoins will have moved to an address that was not backed up. In this case your bitcoins will be permanently lost if you delete the wallet.dat and don't have a more recent backup.
As such it is a good idea to perform backups on a regular basis and keep the most recent few backups.
