Reason: They mean roughly the same thing, and the former tag was populated with 7 images to the latter's 186 as a redundant, duplicate tag.
The latter name is better because having multiple arms (i.e. 2) is actually normal. Creating the alias will prevent multiple arms from being used again.
When I read the title, before actually seeing the thread, the "having multiple arms (i.e. 2) is actually normal" thing came immediately, telepathically, to my mind.
Although I was uncertain about doing this in regards to the arm versions of the tags.
I had tried to sort the arms tag in the past by a similar definitions of multiple_legs and extra_legs, and I went ahead and redid the sorting again. I currently placed under the multiple_arms tag images in which the arms are not "extra." That is, the arms are not "unnatural" in the sense that for a human having an extra arm beyond their initial two is unnatural but for a machine, monster (kind of grey in regards on what to do with humanoid monsters), or a human using a device that has more than one set of arms that is strapped to their body is not out of the ordinary.