As is so often the case, while there may be rules of thumb to be applied here, each staff's circumstances can alter the actual significance of a particular coach's presence. Some examples:
Depending upon the Head Coach's predilection, some staffs conduct their recruiting activities and deliberations very collegially. When that's the case, you can expect the observations and evaluation by an assistant who isn't the RC to carry more weight than in many of the instances described above.
Occasionally, the travel requirements facing the HC and RC will prevent either of them from seeing a particular player who is of very real interest to them. In those cases, they'll frequently rely upon the observations and evaluation made by the assistant whose schedule is available.
There are enough exceptions captured in the above examples that, if I were a player being observed by an assistant other than the RC, I'd treat it every bit as significantly as though I were being seen by the HC or RC.
Need another reason to take it seriously? Even in the most hierarchical of staffs, negative impressions are much more readily accepted and factored into recruiting considerations than positive ones. So, if the assistant who isn't the RC happens to return with negative observations to share, they're virtually certain to find their way into recruiting discussions in a meaningful way.
Finally, a pertinent technicality: In Division I, only 3 coaches can travel for recruiting purposes. In the vast majority of cases, this translates into the volunteer assistant restricting his recruiting activities to on-campus responsibilities and camps/clinics in which he functions as a paid instructor.