Why crit should stay in the game -
It hampers an early lead in the Marksman/Support bottom lane. Think about if crit added a bonus damage % to EVERY auto attack, as is commonly suggested. That would make Marksman vs Marksman trading much more brutal if one is ahead, probably to the point where one team couldn't retaliate at all. The fact that it is (pseudo)random is actually an anti-snowball mechanic as instead of making Marksman champs every bigger balls of stats than they already are.
Obviously finishing your IE before your opponent is a big advantage, but with crit being random there may be a small trade where you don't crit at all. And during this trade, maybe you got crit instead (being ahead of your opponent, they only have the crit cloak in this example). So overall, the trade was a net-loss for you. So instead of a stagnate lane where you would unquestionably dominate your opponent due to having access to a lot more power, the (psuedo)randomness of crit makes the game variable and allows for a comeback by being able to capitalize and react to a varying situation (created by the randomness of crit).
It is a fair point to argue that "I was outperforming my opponent, so I should win - crit being random means I can lose to someone less skilled than myself". That is a logical argument, however it ignores the whole picture. Marksman have always been a huge determining factor in the game, and I would argue that their importance to the team has only increased overall with the Marksman update. If the game didn't allow opportunities for a Marksman to get back into the game, League would become a whole less dynamic - it would become stale. Whoever's botlane got first blood/turret/dragon/IE first wins.
But as I said earlier, if a team that is behind manages to capitalize on a lucky crit and catch up or ever surpass you, that's a good thing. The thing to remember is that you can always do the same back to them!
TL;DR - Crit stops Marksman champions from becoming too oppressive with any sort of lead against other Marksman champions in lane, as well as allows the team who is behind select opportunities to capitalize on a lucky crit to even gain an advantage in game (which the winning team can also do).
Just how I see it.
Oh, and crit isn't entirely random. There's an in-game algorithm that keeps it in check.