Using .tex files from arXiv as a template for new papers
In my opinion (as a senior academic, frequent poster on arXiv, and non-lawyer), it is reasonable and legitimate to make use of the source code in the way that you describe except that &quot;giving credit to the authors of the original paper in the source code&quot; is not enough.  You must give credit in the body of the paper itself.  If you do this, I believe that you would be satisfying any of the Creative Commons licenses used by arXiv.  (In the case of one of the enhanced license choices of ShareAlike or Noncommercial-ShareAlike, you should adhere to the additional conditions.)
If you give credit only in the source code, putting legality aside, you would be trying to avoid publicly acknowledging the way in which you have used the work of the original authors.  That is unethical, in my opinion.  (I expect you would agree that well under 0.001% of arXiv downloads are of source files.  Please do not look for the least possible defensible acknowledgment of the original authors.)
If I were in your field and your action led to increased notational and typographic consistency, I would be happy about that.  It would make papers easier to understand.
[Addition to respond to Nate Eldredge:  You make an interesting point that we do not acknowledge Knuth or Lamport or the authors of various standard LaTeX packages.  It does soften my view.  However, I maintain that there is a difference between the `obviously intended use' when someone publishes a LaTeX package and the rather surprising use of someone accessing source code to use macros.  I still recommend that the original questioner acknowledge the use of source code in a way that is visible in the compiled version of the document.]