Adding snowballs or eggs would only be viable if they were non diamond imo, there's too many other things you should worry about buying before a disposable ranged projectile with diamonds.
gold or iron would be the only thing someone would think about buying them with and then this would also completely ruin early game base rushing and as someone said earlier make camping easier.
its hard enough staying on an island vs 2 respawned people punching you let alone with 2 people spamming snowballs. also from my games played i rarely see situations where i think "snowballs would get me killed right now" or "snowballs would win this bridge cross right now" when heading in towards the late game.
tl;dr
they would either end up being useless or insanely broken their price being a main factor with no middle-ground
and amazing early if iron/gold, useless late