i know routed units do not apply there strength during combat, but when attacked can they play a card??

yes cards can be played. but routed units have no combat value.

in addition if forced to retreat, all routed units are instead destroyed.

Does a routed unit have any value? (not just combat speaking) I seems that I would be better off if they died!?

"He who fights and runs away, lives to fight another day."

Routed units are" UNrouted" at the end of the turn. They may then receive orders, march, and have their normal combat strength.