I've thought about this a little, especially since I just took a test myself, one that I was not initially that interested in taking. I agree 100% with what Nick said, that as a student you do owe something to your sensei, and I'd like to add that I felt a debt of gratitude for my fellow dojo members too. Testing is not only about the person who is up for the grade, but reflects on the sensei and the progress of the dojo in general. I think this is probably why your sensei wants you to test--the dojo is making as much progress as you are.
Also, I think the testing process is an important one to go through just in itself. During a test, a testee has to learn to deal with more pressure and nerves than normal, so it's even more challenging to deal with all those techniques and ukes. Even preparing for the test is a valuable experience, as the extra energy that you will put in will give your aikido a jump start.
I think it's great that you are not that interested in rank, that's the way it should be in my opinion. But I think that testing is an important experience to go through for many other reasons. And besides, if you don't care about the rank, you can just forget about it again after the test!