Yes, the OSSLT is necessary, because it demonstrates that a student is competent. A standardized test that certifies that these students have a basic understanding of reading and writing is a good thing for students and for employers. Employers and institutions of higher education will know that these students are competent, and care hire them with confidence.
In my opinion OSSLT or any other compulsory standardized tests are not necessary. The purpose of the test should be one of determining the capacity and potential of each person, however the standardized tests are usually overtly generalized and insufficiently precise to be able to fit the views and abilities of each individual taking the test. Hence taking the OSSLT is not necessary as it can misrepresent the knowledge one person contains.