Brian Jay Both. Bad modeling practices and software that allows for gaps, spikes and cuts. Some software, um.hm. even have solid validation turned off by default for performance reasons. Then you have the translator issues where people just go with default settings and you end up with crap. But the real rule is "Garbage in. Garbage out."
None of the systems are recyclers. Although NX and Solid Edge do a good job with their synchronous technology to help repair and make changes but that still requires a solid body.
There are two ways to come by your problem. If you are getting dumb surface models, it's literally because of a box somebody is not checking on export, and the errors you get are translation errors.
If you are getting smart models, it's usually because they are complex shapes. Most people doing complex shapes use surfacing tools, because they give more power and control. Because of this, they allow you to make mistakes that would normally not be made by the solid tools. This is not blatantly obvious in SolidWorks, because a lot of the surfacing tools are just 2d versions of the solid tools, but in other programs, the distinction is more important, I think. And even if they don't need it, surfacing types will use surfacing tools, because that's what they are used to.
Surfaces are not commonly used in mechanical design, I think.
Usually people like industrial designers model their parts using surfacing since they want to have nice smooth curves (C1 or C2) which can't be done (might be but it's hard) in solid modeling then they have to deal with issues (not all the time) of converting surfacing to solid models
Other than that, you shouldn't model your part using surfacing
A good point about ID modelling. Also, ID models are typically done with looser tolerances, so when those models are brought into a system that demands tighter tolerances, gaps and slivers result.
I only spend 10% of my time doing surfacing for new product covers - Here is the latest release
The rest of my time, I design parts staring with solid modeling
once a while I have issue with "thicken" the open surfaces - I favor to have a close surfacing body so I can use shell feature for the skin thickness (99% success)
Brian, Any chance you are getting models that have been 3d scanned?
I have seen some bad geometry coming from "automatic" scan to nurbs surfacing.