I am entirely in favor of keeping mobs, both the peaceful and the hostile ones.
Peaceful mobs are especially handy in map 1 where you tend to need to run a lot more, and so they provide that food without having to go for chests in areas where other players might be. Peaceful mobs don't appear a whole lot - actually, I don't really recall seeing any - in map 2, but if they are there they can still be helpful.
Hostile mobs should stay simply because they reinforce the name of the game: Survival Games. Maybe the Hunger Games didn't have zombies or skeletons, but this is completely different. The mobs reinforce that players are supposed to survive as best they can. Spiders and creepers can help weed out players who maybe aren't alert enough or good enough to take on other players (thus not lowering their score), in addition to adding a bit more of a challenge for the better players.
Overall, I say keep them, especially with map 1. Map 1 doesn't have a whole lot of natural threats aside from some occasional falling points/lava holes, and so the hostile mobs help weed out the weaker players. Keeping them in helps makes games run faster and keeps an additional level of variety in the game.