Wait, you might have to leave some of forces at home to protect your homesteads from raiding and invasion? Oh, perish the thought! It's not hard... Leave a couple thousand demesne troops at home while you go off adventuring (and if you can't afford to leave troops at home, maybe wait until you're stronger before invading someone else?).
You misunderstand me. I don't have a problem at all with having to pay the consequences if you strip your homeland of troops and sail off into the sunset, nor do I in the slightest want the raiders to be nerfed because, well, see the first part. NOT being able to just march off your entire army in one direction and expect anybody in the other direction to just sit tight until you come home is one of the things that make CKII stand out and makes it a LOT more realistic to play. Because when my AI neighbor marches off, that's when
I strike, so the AI should use the same logic.
That's all great. I just wish that there was a more elegant way of doing it than constantly franti-clicking between Jerusalem and Scotland like an RTS on steroids, going "raise, move, oh wait, other end of map because battle ended, follow enemy on other end, click back, enemy moved, follow, what's that? somebody raised another army on the other end?, wait, enemy at home dead, disband, back to foreign lands... etc. etc. etc." And do this every five minutes.
The best I've come up with, as I mentioned originally, is to just give all of the coastal provinces to vassals because A) I don't care if they get beaten up a bit, keeps them in line and B) as you said, a three county Duke vassal is perfectly capable of policing his Duchy without me micromanaging everything every five seconds on three different areas of the map.
So no, I absolutely DON'T want the raiders to be nerfed away, that would make the game incredibly boring and ahistorical.