Sapped a lot of their resources and made them look vulnerable to other enemies. You may be right.
The Caledonians and Celts gave the Romans fits. I think of all of the Roman Empire, Brittannia resisted the hardest. An interesting fact-sometime around 200 AD, a Roman Dux was transferred to Hadrian's Wall from a Danube stronghold to "restore order". His name was Artorius Castus. An inscription was found his sarcophogus detailing his exploits at the wall. He was fair to the locals, loved by the men he led and his section of the Wall was relativley free of trouble during his command. It is my belief he was the basis for King Arthur. The Venerable Bede, a 6th century chronicler mentions a great battle won by an Artorius, who was fair, honest and loved by his men. A great take on this theory can be found in "King Arthur" with Clive Owen. A great movie, IMHO..