Should a teacher only use technology to assess student learning? Why or why not?
It is difficult to know exactly what you mean by using "only using technology" for student assessment, but I am assuming you mean that a teacher would be evaluating student performance through only technological means such as "machine-graded" tests or using software programs to assess essays. Theoretically, technology is simply a tool, not good or bad in and of itself, but in my opinion, while it can be of value in education, it would never be my first choice as a means of assessment.
There are two kinds of assessment a teacher needs to be concerned with, formative and summative. The first is meant to help the teacher see whether or not a student is on the right path, which allows the teacher to address deficiencies quickly, before it is too late for the student to learn. Without formative assessment, students do get left behind. Formative assessment is not necessarily formal at all. For example, the teacher may note the quality of participation in classroom discussion or simply whether or not the student seems engaged in learning. Summative assessment is at the end of a course or a unit, meant to assess the student's learning for the entire period. This is officially for the purpose of grading, but it also allows the teacher to assess his or her own performance as a teacher, and this can lead to improvements in teaching in the future. Students who fail are an indication of the teacher's failure, not just the student's.
For formal assessments of either type, reliance on technology presupposes tests that are objective in nature, for example, multiple choice or fill in the blank kinds of tests, or student writing that is assessed through some sort of software. I am strongly opposed to the former, and completely opposed to the latter. Objective testing generally assesses only the student's ability to remember, usually not a particularly good indicator of real learning. So it is not so much that I have anything against technology, but rather that technology limits the nature of the testing one can do. As far as software to assess student writing is concerned, I am completely underwhelmed by what I have seen thus far. Microsoft Word's grammar checking function is riddled with problems, and we are nowhere near a point at which software could properly address higher order concerns such as organization, content, or competence in any discipline. I don't even want to think about the limitations such software has in terms of providing feedback to the student. I am teaching an on-line course right now that provides such software, and fortunately, I am not required to use it. A human being must assess student writing, to assess the student's progress or lack thereof, to provide meaningful feedback that promotes learning and that encourages the student to keep up the good work or to do better.
One use of technology assessment in teaching that I find particularly abhorrent is the requirement that students use various programs to determine whether or not they have plagiarized. This is an abdication of the teacher's responsibility to teach about academic honesty, encouraging students to not be concerned about what they are doing unless and until they are "caught" by software. Every student is capable of figuring out whether he or she has plagiarized, if the student has been properly taught what this means. And every teacher is capable of spotting plagiarism. It practically leaps off the page. I find this trend in education deeply troubling.
While I have taught some on-line courses that were "created" by others and that include technologically-based objective testing, I do not rely on this exclusively for either formative or summative assessment, and for on-ground courses, I do not rely upon technology for any testing at all. I would be quite unhappy with myself as a teacher if I had to use technological means only to assess my students. This is, of course, my opinion, and you may very well find that others do not think or feel the same way.