Both lower_level_thinking worksheets and too many (even high quality) worksheets can hold students back by not providing stimulus and challenge. Studies have shown for years (just do a Google search!) that children learn best through active involvement and real_life experiences. Virtually every teacher knows that children learn to read by being exposed to books and by being read to, NOT by completing worksheets. The same applies to all areas of learning.
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.