well I dont think the scuds are at all a match for the arrow system, but iirc not all of isreal is protected, and many arrow components are US sourced, and the current political bunch here have started blocking American company from exporting weapons to Israel....so there is always a possibility of running out of interceptors, and as jim already mentioned, scuds are cheap, so its not completely unfeasible for Syria/Hezbollah to simply wear down Israeli missile defense capabilities by simply forcing them to expend all their missiles.
And despite the simple nature of scuds, they are a substantial improvement over the unguided rockets Hezbollah has used in the past.