I always thought that it depends on the engine and platform if intersecting polys are a problem. Most modern engines for pc games don't have a problem with this I think.
Some engines you will get Z-fighting with interpenetrated mesh. Another thing to consider is vertex lighting looks better with a contiguous (airtight) mesh. IMO - If you are learning I think it's better to try and make your models airtight. Intersected mesh can be more efficient where polygon count is a concern, however.…
I've got a question to ask of you guys that was specificly brought into focus after reading the modelling section of the recent book "The Game Artist's Guide to Maya". That question concerns the pros and cons of low poly contiguous meshes vs. combined objects. In the modelling section of the book there's construction…
As a rule of thumb use segmented penetrating meshes primarily for environments, and use seamless single meshes for characters. Although there are always exceptions to the rule, I believe some crappy FPS engines only allow seamless meshes as well they backface cull so you have to cap the ends of pipes, also most character…
For vertex lighting, and to avoid Z-fighting, contiguous is better. That being said, those are not always issues that you will need to be concerned with, depending on the game engine. Intersecting geometry can save you a ton of polygons, and can just make things a lot easier to edit. Particularly in the case of hard edged…