While Riven is not being played in NA LCS she has been played in other regions such as China and Korea to great effect as a counter pick. If I recall correctly Smeb from the KOO Tigers got a Penta-kill this season against the Jin Air Green Wings. The issue with Riven is that many situation make other options more appealing due to assorted meta reasons, but that is not to say she is not played on the competitive level.
The general strategy of buffing champions not picked in the LCS has several issue on it own, discounting the success or lack of for Riven, because there are champions that come to prominence in other competitive regions (which always prove more successful than NA during international play) before NA picks them up.
Another issue is champion pool depth. If a team picks, lets say for example, the top four to six champions they want for a role to practice, there is an inherent advantage to practicing an overlapping champion pool so your players can 'steal' power picks from other players who are good with them.
To be clear I am not weighing in on whether or not Riven or any other amount of champions are balanced, but rather I am saying there are several inherent flaws in balancing around the LCS.