When my progressive diagnosis was given, I asked about ALS. The doctor told me that ALS is easily detected (easier than MS) and I did not have it. The EMG might be their last try at dismissing MS (IDK), but I hope someone can enlighten you.
If you have run through the testing criteria, I would not worry about anything else. When I was dropping into really bad, bed ridden shape, I thought I could have malignant MS or ALS. It was an unnecessary worry, however, and I am glad I asked the doctor.
Yes, I know of several people with MS who've had this test, usually combined with Nerve Conduction Testing (which they will probably do too).
If you do not have lesions, this is one way they can determine the extent of your nerve and/or muscle damage. Depending on the results, they may be able to narrow down the likely CAUSE, including MS, ALS, etc. (there are dozens of causes, but the apparent "problem" does help narrow down the cause).