Surface Modeling Basics and Troubleshooting

 From:  Michael Gibson
4751.14 In reply to 4751.1 
Also a couple more previous discussions on why you usually want to make the top and bottom surfaces like that as extrusions and not with a pinched-together surface:

These have some more illustrations and explanation:

You can make a pinched-together self-intersecting "bad" surface if the only thing you want to do with it after that is just look at it or export it to a rendering program.

But if you want to do more modeling operations on it, particularly ones that involve surface/surface intersection like booleans or filleting, then you'll run into a lot of problems.

Just being pinched together itself is not really the problem - it's how things are shaped in the neighborhood of the pinching. It can be possible to create an ok surface that way as long as you're very careful about how the curves are shaped in the area where they touch, they should be exactly symmetrical and have a shared tangent and especially don't have any inward curving shape like this:

Even the slightest bit of something with that kind of shape will make a bad surface that folds over itself in that tip region. It's so easy for that to happen that it's really best to plan to make such things as trimmed extrusions from the beginning though.

- Michael