I understand having randomizers for the kingdom cards because that's how you randomly choose them, but I find it odd that there is a randomizer for every card including the basic Curses, Coins and Victory cards. Why are they there?
They are there to mark each pile. The game ends when 3 piles are empty. Once you see 3 randomizer cards showing, the game is over.
This is on page 5 of the rules:
The way my group lays out the Kingdom cards is organized by cost, so it is not always obvious that there is a pile missing. The kingdom cards help remind us.
I agree with your comment that the Randomizer cards are a bit too similar to the normal cards, one solution is to put them into different colored sleeves than your regular cards. Or perhaps, just sleeve the randomizer cards.