Why would there be two RNAV waypoints co-located with a VOR?
Your Questions: Why do SOHOW and USIRE exist? And in particular why do they both exist?
Why do they exist: SOHOW and USIRE are published Waypoints on RNAV STARs into Gary, Ind., and Indianapolis, Ind., respectively. They exist and are published because they are part of the procedure being flown (as assigned by ATC).
Why do they both exist: Even though they are in extremely close proximity to each other in space, they are not physically located anywhere. The fact that both of these RNAV Waypoints are only &quot;points in space&quot; (geographic coordinates in a computer) reflects the value and flexibility of Area Navigation.  Perhaps both STARs (to Gary and Indianapolis) could have used either SOHOW or USIRE, but there is no physical reason to do this.  Also, since each of the Waypoints (SOHOW and USIRE) are part of different STARs going to different airports, there is less opportunity for confusion as to which procedure is being flown.
As a note, you mention in reason number 1 of your post (your Counterargument) that Spinner Vortac is a waypoint on the NATCA (RNAV) SID.  Spinner is actually a &quot;transition&quot; on the NATCA SID, which technically ends at NATCA.  See image below.
Good questions.