I have been asked numerous times how to do a proper assessment and whether self-assessments can do the job by themselves. I do not believe that self-assessment should ever even be considered to replace other forms of assessment. The subjectivity of self-assessment, as well as the level of expertise from person to person varies so widely that using this exclusively would be a catastrophe. I do, however, think that self-assessment is a great tool. As Harrington (1995) mentions that most assessment tools, like standardized tests, are too generalized and not specific enough to the task at hand.
As most assessments are used as a pre-screening tool for employment purposes, using self-assessment would not be a very valid instrument to use for hiring practices. There is little incentive to be honest in this case with your skills. As a teacher who is trying to teach a college success course to new students where they try to understand their strengths and weaknesses, I can tell you that people drastically underestimate their limitations. For some reason, everyone deems themself an expert on everything despite massive evidence to the contrary. If training were performed to more accurately self-assess or if self-assessment were used as it is now as a supplement to other assessments, then this would be ok. If it were used as a substitution I do not see how accurate assessments would be attained and useful information gathered.
Harrington, T. (1995). Assessment of abilities (ERIC DIGEST). Greensboro. NC: Eric Clearinghouse on Counseling and Student Services, from ED389960).