Limiting the number of games being launched will ensure prevention of naughty players from spamming bots.
Blizzard wrote on their official forums:
While we regularly take action against accounts for the use of unauthorized third-party programs and bots, this additional measure will help us further preserve and protect the integrity of the game and economy in between ban waves.
In case you’re facing an error message: “Input Limit Reached” and you’re not doing anything that goes against Blizzard’s game rules then be sure to let them know.
This information will help us ensure the limit minimally impacts legitimate players while still protecting the game against bots.
Our goal is to help ensure that Diablo 3 continues to be a fun gaming environment for all of our players, and we’re looking forward to hearing your feedback on this change once it goes live.