Someone once asked an old and grizzled typographer why he preferred justified typesetting. His answer was simple: it was the way he was taught to set type on a Linotype machine as a boy. When asked why he supposed he was taught to set justified type on the Linotype, his reply was that was, "That was the way the old-timers preferred to set foundry type." When pressed, why he thought the old-timers preferred justified composition, he pondered for a moment and guessed that it was because the first typesetting, the Gutenberg Bible, had been set that way. When further pressed about the Gutenberg Bible's arrangement, he thought for a moment longer and then beamed. His answer was: "Because the scribes in ancient monasteries produced their manuscripts that way."
So there we have it, justified typesetting has been a prominent composition format for so long because God likes it!
Unfortunately, not everyone is in agreement with God's typographic taste. In fact, one of the most fiercely argued questions in typography is whether justified composition is better than unjustified. One faction of typophiles will tell you that justified composition aids readability, is more pleasing to the eye, and generally is better typography.
Another faction will tell you that unjustified is better--for the same reasons! The truth is: neither one is inherently better than the other. Justified and unjustified composition are both equally readable, and can be good (or poor) typography depending on how they are handled.
Why the controversy? Because both formats can cause typographic problems if handled poorly, and many experts choose to believe the problems are intrinsic to the typesetting style. If you are aware of the possible problems and avoid them, you can set beautiful and effective typography justified or unjustified.
Justified Typography
If you prefer justified typography you have tradition, and the likes of the old typographer, on your side. Probably the real reason for the popularity of a justified format is because it is very predictable, and creates simple geometric shapes that are easy to fit into a design grid. Justified typography creates order and is essentially "no-brainer" typography. Once the composition equipment has been put in a justified mode, the person doing the key-stroking does not have to make the line-ending decisions. The equipment logic determines where to stop the line and automatically adjusts the interword and/or inter-character spaces to equal the predetermined measure.
The major problem with setting justified copy is the risk of creating excessive interword spacing. Many times the workstation or output device is presented with remaining space on the line, but not enough to set the next word or a hyphenation of it. Therefore, the word ends up on the next line causing the previous line to be spaced out with additional interword spacing. The result can be too much word spacing. Short line measures are especially difficult to set flush left and right. A look at many newspaper columns will show you excessive word space and "rivers" of white running through the copy.
The longer the line measure the less this problem occurs. Books and magazines that are set justified can have tighter word spacing since they generally use wider columns of type. Another problem with justified copy is that it is, well, boring. Sure, those neat blocks of copy create an orderly, consistent design; they also can create a dull page--or pages if you are producing a book, magazine, or brochure.
Some have suggested that boring typography is bad typography. No studies substantiate this theory, but it is reasonable to assume that while boring may be benign, it certainly isn't ideal.
Unjustified Typography
The biggest advantage of unjustified typesetting is the ability to control word spacing. You can have tight, even word spacing in any length line. Not only does consistently tight word spacing look better, it's also an important aspect of efficient and legible typography. Tight word spaces speeds up the reading process and allows the reader to absorb thoughts and phrases rather than individual words, which helps to maintain high levels of comprehension.
Some people also believe that rag-right composition is more inviting to the reader; that its somewhat casual appearance encourages reader involvement. Truth be known, unless there is excessive white space between words, readers don't care, or notice, whether they are reading justified or rag-right columns of text. Numerous studies, under a variety of conditions, continue to prove this typographic fact.
The disadvantage of unjustified typesetting is that it can be difficult to produce and still remain high-quality typography. Very long lines followed by very short ones can cause awkward shapes that are not inviting to the eye. Ideally, unjustified composition should appear to be optically justified. And if the right-hand edge of a column does describe a shape, it should be convex rather than concave. This means that the person responsible for setting the type must use a esthetics when making line ending decisions. In many systems, the machine logic determines where a line ends. The problem is that logical decisions are not necessarily also esthetic decisions. The result is that the person creating the graphic communication is forced to carefully review the first set of production proofs and re-break lines of copy to correct an unattractive set of line endings. Sometimes, copy should even be rewritten to facilitate this process. (This kind of stuff happens a lot in the production of U&lc.) Unfortunately, the real world rarely provides the graphic communicator the luxury of manually re-breaking lines--let alone the power to request that a writer reconstruct his or her sentences! The result: something less than ideal.
Paragraphing can also cause visual problems. If the copy being typeset has a number of short blocks containing paragraph indents, the finished piece, if it is set justified, can look as though it was set ragged left and right.
Which is more desirable? Only you can answer this depending on your personal preference and the particular job application. Either will reward you with high quality, effective typography. Just be wary of excessive word spacing when setting justified and very uneven line lengths when setting unjustified.