The downside is playing against bots. If a server shows 128/128 players, I want 128 players, not join the server and figure out 100 of them are bots.Who cares if bots are optional or not? There's literally no downside to them being there until the server fills up or new plays replace others who left.
The is Opt Out by default is better as most players won't even change the setting and it will mean players like me who will opt out will have far less people to play with by default.
Opt Out by default is better overall and allows those who want to play with PC players the option.
Look at that awesome kill I just made!! Oh.. I killed a bot.