Why wipe all BTC from an imported address? I've read a couple blog entries and tutorials on how to spend BTC that are "stored" on a piece of paper.
They all suggested importing the private key into e.g. blockchain.info's wallet (or any equivalent)
and wipe ALL coins from that address. But why should I not continue using it?
Once your import the key, the key now exists in two places: your paper and your wallet.  Because of this, you've lost the advantage that a paper wallet has for cold storage.  What you now have is a paper backup of one of the keys stored in your hot wallet.  Any risks of a hot wallet now also apply to your paper wallet.

In addition, there is the inherent privacy issues with re-using a Bitcoin address, regardless of how that wallet is stored.  There aren't many advantages to re-using addresses, and this case is no exception.  If you still need a paper wallet after importing one, you should really create a new one.