I've never had an external strainer. A friend had them on his power cruiser, and we had to shut down one engine when it overheated due to barnacles on the external strainer. That being said, I had to repeatedly shut down my engine and clean the internal strainer one day while motoring across Florida Bay, when sea grass kept blocking water flow. At least I could get at my strainer without diving.
Everybody has their own experiences. Like you, Tony, I had frequent grass clogs in my internal Groco strainers for both my engine and genset. More for the genset- I think it was because I was not moving and anything that happened to float by got sucked in.
I would never install the one-piece strainer and thru-hull like the OP's picture shows because it is too hard to clean out. In fact I removed onefor my A/C intake. In post #2 above, a previous thread was referenced and in that thread I have pictures of my hinged-cover external strainers. Since installing those strainers with holes, not slots, my internal strainer receives almost nothing. I literally clean my internal strainer once per season and only take out a couple short pieces of grass.
So the bottom line is that not all external strainers are equal. As Stu likes to say, "Your boat, your choice".