Black bars can mean as originally intended to be viewed
Adding on to swiftaw response, you might want to look into aspect ratio and really give yourself a good education on it. The idea of a widescreen TV is not necessarily to perfectly fill the screen top to bottom & left to right, but to offer a good compromise between classic TV content (shot at 4:3 (basically about square) and the widest widescreen aspect ratio films (shot at up to 2.39+:1 (very rectangular widescreen).
The former is most TV programming shot prior to TV shows switching over to a widescreen shoot (for futureproofing the shows). The latter is not even the max (Ben Hur for example is 2.76:1, and some old film from 1927 was shot in 4:1 (super, super widescreen, black bars would dominate the screen, with only a thin strip of "movie").
As a point of comparison, your 16:9 widescreen is perfectly filled when the source aspect ratio is approx. 1.78:1.
Now, you can crop your rips to fit this aspect ratio, but then you are throwing away picture to make it fit when the source aspect ratio is greater than 1.78:1. Some films take advantage of aspect ratio by putting important elements at the very edges. So if you crop to make it fit, you might cut off important parts of the picture.
"Purists" want "original aspect ratio" so that they are getting the full image as the director intended. If the aspect ratio is greater than 1.78:1, this means black bars above and below the picture. How much greater than 1.78 dictates the thickness of the bars.
"Full screen" DVDs are meant to feed the same "aggravation" ("I want to use my
whole screen") driver for people with 4:3 TVs who don't want black bars on screen when watching a "widescreen" DVD. Those DVD's fill the 4:3 screen. But, they do that by either cropping off chunks of a wider aspect ratio film and/or using a concept called "pan & scan" in which different parts of a wider screen source are being "framed" to fit into a 4:3 picture.
In other words, imagine a widescreen frame from a movie as a rectangle (that holds the full width of the original shoot as intended by the director). Now imagine laying a square on top of that rectangle that is just as tall but not as wide. In "pan & scan" the square would slide left & right on the rectangle in an attempt to capture what the "full screen" editor thinks is the most important elements to have on (the 4x3) screen at the moment.
So, if a director was shooting a western shootout and the good guy was at one (very) edge of the wide aspect screen, and the bad guy was at the other edge, the pan & scan editor would be shifting back & forth to show one or the other as that editor sees fit (typically following dialogue). If you cropped this same widescreen film to "fit" your screen, you might very well chop off both cowboys, such that you might see only whatever is shown between them. In this scenario, you might be staring at a general store for a few minutes and hearing dialogue, but seeing neither the good or bad guy. They might even draw and one may fall, but all you'll still see is that general store.
Similarly, if you go to the movies, you'll notice that most theaters frame their screen with curtains. If you peeked behind the curtains, you'll typically notice that there is more screen there (basically the curtains are acting as "black bars"). How wide the curtains are opened is adjusted based on the aspect ratio of the film to be shown. The film might be scaled so that it is always the right height for the screen (though some theaters have vertically adjustable curtains above the screen too), so that the width (of how much screen to show) is all that needs adjusting for each film. This is a nice trick to make it look like the film is always perfectly filling the screen, when in reality the physical dimension of the (shown) surface of the screen is adjusted to "fit" the aspect ratio of the film.
Some good, succinct info on aspect ratio can be reviewed at:
http://en.wikipedia.org/wiki/Aspect_ratio_(image)
