One must do that with which they're comfortable. I'm not telling you not to use an external strainer--just that I don't use one. Here is my reasoning; draw your own conclusions.
If I'm going to have a clog, I'd like it where I can get at it, not outside the boat. I think you see so many external strainers because original builders/outfitters are looking for the inexpensive and easy option rather than springing for the higher cost (and space requirements) of an excellent internal strainer. Given how miserably cramped sailboat engine rooms tend to be, it's easy enough--if indefensible--to see why some might not favor large, easily-accessible internal strainers, though this is shortsighted.
I just don't see what an external strainer does, other than clog inordinately and cause issues that are challenging to resolve. I don't much like swimming even in nice water; I'd certainly prefer to avoid it at all costs in icy northern waters. All this being said, eliminating an external strainer means that a good internal strainer becomes critical. It should be mounted where it's easy to get to and inspect and service.
Any clog is inconvenient, but if something gets sucked into your hose, at least you can close the seacock, remove the hose, and clear it out--or even throw in a new hose (everybody has spare lengths of hose on board, right?). If something clogged my intake, It'd be easy to clear the through hull from inside the boat if I removed the hose from inside, since there's no strainer on the outside.
I think those external strainers clog with mussels, little shrimp, and weed, not to mention layers of bottom paint over time. I check my internal strainer daily and clean when necessary--which, frankly, is almost never. More often, I clean it out just as a matter of course if there's even a small amount int here. I steer around floating seaweed and debris whenever possible. Two years ago, I never opened the strainer all season, and this is all that was in it at haulout. Now, I think this was the first time I'd never had to open it all season, but still.
![Image](http://www.triton381.com/image08/october08/seastrainer-100408.jpg)