When I first bought my current boat all it had was the external strainer. Getting the grass out of the heat exchanger while anchored was interesting. Just to be safe I sent my wife ashore so I could speak freely. Since then I've been able to catch the grass and other crap in the basket and haven't had to open the heat exchanger. Heaven forbid that you get any grass in the transmission cooler. I have also had grass clog the hose from the sea cock to the basket, again the basket kept it out of the engine.

Remember Mr. Murphy (of Murphy's Law fame) founded the Optimist Club.