Hello Hancock, first of all, welcome to deskdemon!

I temped for several years. It certainly is not for everyone, but I can tell you that it ultimately led to MANY job offers for me. However, there is no doubt that temporary employees are often second class within the company and are usually left out of most employee related things.
I see a couple of issues going on here.
First, a question: why are you doing the temping? If it is to make money while you continue to look for another job that is a different perspective than you are doing it to get your foot in the door with a company and convert to permanent. If it is the first, then you just need to carry on until the right job comes along. If it is the latter, then you should communicate with the temp agency that this job's responsibilities are not matching the description you were given in advance of the job and suggest that they find someone else to complete the assignment.
You do need to walk a careful line with this if you wish to continue being placed by the service. They won't allow you to walk out on too many assignments before they stop trying to place you at all.