First, thank you for taking time out to write recommendation letters for students. Rest assured your efforts can make a huge difference in the admissions process.
Professional schools are trying to get to know the student through your letters. They are extremely important in the application process and carry a lot of weight with admissions committees. A two-line letter is not generally very helpful and often ultimately hurts students as they were asked to request letters from people who know them. Your efforts to write a thoughtful letters that speak to the academic and personal qualities of the student are greatly appreciated!