I think the reason Khaled has't added such a feature already is because of the number of different spoken languages that make up the IRC community. If a swear filter were hardcoded in to mIRC, Khaled would have to filter every one of those languages - and that's no mean feat.
The advantage of scripting such a feature is that it can be taylored to your specific language easily and only you decide what gets filtered and what doesn't.
I personally don't understand the reasoning behind people's getting offended by swearing as they're merely just ordinary words. I mean, would you get upset if someone used a different word with exactly the same meaning as the one that offends you? Such as 'sex' for the 'F-word', for example. At any rate, taking offense to swearing only gives the swearer power as they now have a weapon to use against you - a weapon that just resides in your head.
So, to draw this meandering post to a close, the upshot is that it's just not practical to hardcode a swear filter in to mIRC