“There may be many reasons to use higher quality lenses, but increasing/decreasing DOF is not on of them”
While technically there are only two things that affect DOF (focal length and aperture), cheap lenses like those found on almost every pro-sumer camcorder have poor optics compared to more expensive lenses and this affects apparent DOF because they are not collimated to the precision that 16 or 35mm lenses. Since the optics are not as precise they appear to have a greater DOF because the overall image is not as sharp therefore the tolerances on the circle of confusion are less and a wider acceptable range appears to the CCD. Think of it like focusing the light of the sun through a magnifying glass. On a good one, you can get a very small circle with clearly defined edges before you start getting distortion in the shape and it starts to look like a figure 8. If you use a cheap plastic magnifying glass, it is very hard to get a clean circle, and the edges are not distinct even when the circle of light is relatively large. This isn’t a precise analogy but it’ll give you a good idea. Or, do an A/B comparison with a video lens and a film lens at the same focal length and f-stop on a Siemens Star and you’ll see a noticeable difference in picture clarity. This directly affects the apparent DOF, which is obvious when you think about it. If overall clarity is lost, then the delineation between the areas of soft focus and hard focus will be harder to determine.