Random crits give new players a chance to win against old good players, which is a great thing as it makes the game less hostile. They aren't even unbalanced against good players as the more damage you do, the more likely you are to crit. So it enables those amazing killing sprees you sometimes get.
This. Valve know what they're doing. Almost all of their game design choices have solid reasons behind them (except the dalokohs bar...), crits are no exception.
Having played on critless servers, the biggest difference is once a game tips toward one side, it doesn't change often. Random crits allow those magical moments where one isolated soldier goes from getting chased by 4 enemies to proudly standing on top of his very own pile of ludicrous gibs, or when a heavy and medic hold a point just long enough for the next respawn wave to come through.
Basically, random crits adds hope for the losing side, and a little danger (and thus interest) for the winning side.
And lets not forget the RNG for crits is biased system within TF2.
If you're having a long life, and getting more kills, you're going to get more crits. So that high game play continues.
If you're having bad Kill to Death Ratio's you're also going to get more crits.
I wouldn't be surprised if the crits were also adjusted based how the round is going. On CTP maps, that the attackers seems to me, to get more crits as time counts down.
This, is why no crit weapons are such an interesting balancing mechanic.