I've never seen anything to indicate that the wall played any important role after the Romans left the British Isles. As I recall there were fairly regular instances over the following centuries of an army coming from the north to raid. When this happened an army was raised and marched to meet the Scots. The Scots were defeated in battle (usually) and then ran back north.

Some of the more prominent forts and such were kept in use as local population centers after the Romans left, but widespread use and garrisoning ended. As a whole the wall would have required the kind of resources that only a strong central power base could supply in order to remain in good repair. The wall was mainly a tool for a strong centralized authority to exercise control over territory. When the Romans left so did the reason for it's existence.

Within a century or two the wall was seen more as a source of building material than defense. As late as the eighteenth century, stonework from the wall was used to create a road for English troops to head north into Scotland.