Students can certainly benefit from practicing new skills and concepts on paper. From letters and numbers to report summary formats, worksheets can provide students with a framework for practice _ an avenue for synthesizing new information in their brains. Well designed worksheets can also give students a platform for expressing creative ideas and reaching towards higher levels of thinking.
So what is the concern? There are three big concerns, actually. Our teacher "caution light" should start blinking if we are using lower_level_thinking worksheets (Example: "Check the box next to the correct answer."), if we are using too many worksheets or if we are using worksheets as classroom busy work.