Define "side imbalance".
Ideally an arena should feature sides with roughly equal numbers. If you see one or two sides with significantly larger numbers than the others, that's imbalanced. If you see wild fluctuations in the number and proportion of players on sides, that's also problematic since it makes it difficult for HTC to implement ways to balance the numbers over the long term. That is, if you have a
long term imbalance where one side always seems to have more players than another, you can remedy that through ENY, by putting new players on the sides with lower numbers, etc. If those imbalances shift wildly over the course of a night, as I suspect they probably did, such solutions have no real impact on balance.