Additive Vs Subtractive

I've been playing around with the RoboBlitz Editor. I've previously worked on (if it could really be called that) maps using the previous UnreadEd editor and using the Source engine. This caused a couple of hiccups when I started using this system as I expect I was working on a subtractive level like I would with the previous UnrealEd but it turns out it defaults to additive.

For those of you who have no idea about what I'm talking about but a desire to know think of subtractive as carving air out of solid rock rather than additive which is creating rock in air. Subtractive reduces the chances of hall of mirror effects and people "falling" through the walls as behind the air their is always solid mass. I don't see many disadvantages to subtractive modelling in terms of performance and stuff but I'd like to know if there are. I know that RoboBlitz itself used additive creation instead. It's interesting that Epic included both types into their engine. Maybe to try and bring in people used to additive building? I'd assume it is possible to include both types in the same game (though not the same map, obviously) and maybe there is benefits to swapping between them.

I liked Valve having designer notes in Half Life 2: Episode One it was interesting to see how they went about things or what they changed due to things being impractical. For me Valve remains one of the most polished designers I can think of. Their games draw you in so well.

Anyway next in my Unreal 3 engine playing is scripting. I'm going to try writing a bit of a tutorial while playing around with it. Hopefully I'll have something to talk about in the next three weeks or so.

In other, unrelated news, I hear that Settlers of Catan for XBox Live is coming out soon. I'll looking forward to it :D.