It really depends on the relative utility of that $5 to you. For some it would be a barrier, in that they would have free time that could have potentially been spent watching a movie but they didn't want to spend the $5. For others, the $5 is such a small amount that the barrier is time.
Personally, the biggest constraint on my ability to watch the large amount of films that are on my "to view" list is not the cost of the individual rentals but the availability of time in which I can actually devote to sitting down to watch a 90-, 120-, 150-, or 180-minute long film.
The ability to split up my viewings over multiple sessions helps but with the way that most online streaming rental services work for current films, 24 or 48 hour windows, I've had a number of films I've rented age-out before I've had a chance to finish them.