According to the Wikipedia definition (as of April 21, 2013), an idiolect is “…a variety of language that is unique to a person, as manifested by the patterns of vocabulary, grammar, and pronunciation that he or she uses.” This accords to the definition I learned in graduate school many years ago.
In my writing, I try to make as much use of idiolects and personal speech patterns as possible in order to distinguish speakers in sometimes lengthy conversations so that I can omit boring, repetitious attributions such as “he said”. I feel this also adds a sort of flavor to the story, because the way a person speaks tells something about the speaker in terms of emotions, psychology, and background among other things. Using idiolects adds a layer of subtle complexity to a story.
An example of this from my past is that of a college friend named Mike. One of Mike’s pet expressions was “Whatever!”, which he used often in a sort of sympathetic exasperation when someone persisted in doing something Mike thought stupid in spite of his advice to the contrary. On those occasions, he would chuckle and say “Whatever!” and walk away with a grin that said he would have fun seeing the outcome. If I were to write down a conversation between myself, Mike, and several of our friends, you could tell when Mike was speaking by his frequent use of the “Whatever!”, which the rest of us seldom used.
Used carefully and sparingly, an idiolect can be a subtle motif about each character that the author can use to remind the reader of some facet of the character at critical moments.