I also studied the impact of n/N
(n is the current slide, and N is the total).
Here are the benefits I see:

a. The number n helps,
   at the presentation end,
   when someone likes to ask a question
   related to some past slide;
   the jump to that slide
   is made much quickly,
   using the numeric pointer (19),
   rather than a semantic pointer
   {"please go back to the slides
   in which you explain tra-la-la").

   However, if only n is visible all the time,
   and nobody knows what is N,
   people tend to think negatively:
   they assume that the number of slides will be ZILLION,
   and they easily go into the sleep mode
   (especially the students).

b. If both are visible all the time
   one knows how much suffering
   to expect till the end :)

   People feel better
   if the know what is going on
   and when what finishes.

   If both are visible at the same time,
   the concentration of students goes UP
   towards the END of the presentation,
   for one simple reason:

   If they see n=19,
   and nothing else,
   they assume that N=1000,
   and they go to sleep.

   if they see 19/24,
   they know that only 5 more slides remain,
   and they wake up,
   in order not to miss something important.

   This pattern of behavior
   is quite frequent among students.

c. In neither n nor N are present,
   I personally feel offended!