The problem with this, though, is that once you hit the enemy cap, new things will spawn in very slowly (you must wait for the game to decide to despawn one of the current enemies and that can take awhile) and you are forced to either attempt to despawn the mobs already there, or kill some of them (which will add to the event counter).
Not hard at all to just run off the screen and spawn in new enemies. Regardless, my point stands; NOT killing Dutchmen is actually the counter-productive technique, rather than the other way around.
Also the advantage of invasions is that they aren't timed so you could run out of range of the invasion to despawn the current Pirate enemies then come back to improve the chances of spawning a Dutchman and not progressing the Invasion progress by killing normal Pirates.
I actually wasn't aware that the Dutchman has a higher chance of dropping rare Pirate Items. (I thought it could only drop the Trophy). So the Dutchman makes it easier to get rare Pirate loot compared to Pre-1.3?
Either way, I agree that 50% would be too high a drop rate, but 5-10% sounds a little more reasonable to allow for a better chance to get rare Pirate items while still keeping them rare. (unless the Dutchman already has a drop rate around that amount for later in the invasion.)