The portal clearing game seems kind of pointless at present from an overall game experience. If we could delay the invasions in some way by clearing the portals it would provide some invasion relief and get more people actually clearing the portals from the land.
I would suggest and ever decreasing benefit as you clear more portals before an invasion as obviously we would still want the invasions to happen eventually. Either that or reduce the waves/number of enemies based on the number of portals cleared.