My samples have been frozen and thawed multiple times, is it still practical to test them on a multiplex assay?

Yes and No. The answer to this question is dependent upon many factors. The main questions to ask are: How long have the samples been at room temperature? How difficult is it to collect new samples? What analytes are you assaying? Is the data for a publication or a preliminary experiment? Please be advised that inflammatory analytes are known to degrade easier. Also, mouse and rat blood based samples tend to degrade faster than human samples at room temp because they have higher protease levels.