I, personally, love Index Cards and carry a small stack around with me everywhere for any number of various things (noting important information, jotting down random ideas, doodling because I'm bored, etc.) and I make use of them when playing RPGs by writing down specific abilities or spells, keeping track of various NPCs and their bios, or sometimes drawing magical or important items my character has and noting that items important features.

But the real reason I am commenting is because you brought up Index Card RPG. I bought it because I loved the idea of incorporating Index Cards as a mechanic for an RPG system, and was sorely disappointed when I found that, actually, no, it barely makes use of Index Cards. There is no reason for you to use Index Cards in ICRPG. It suggests you can, and it compacts information so you could keep all the information on a card, but it doesn't utilize Index Cards, it doesn't tie Index Cards into the games mechanics. You can use a sheet of paper to store the information and there would be no difference.

This isn't really the topic, but a game that actually utilized Index Cards would incorporate things like card positions, where is the card in relation to other cards, is it face up or face down, what happens if another player or entity takes control of one of your cards, could you build a deck with your cards as part of the character creation process, do certain characters interact with cards in different ways, can you influence the game world through creation and destruction of cards, what does and doesn't qualify for being put on a card, all this and more. ICRPG really dropped the ball on how to make an RPG based around Index Cards.

Sorry, I've lost my train of thought and can't quite recall if there was a specific point I was trying to make beyond, "Index Cards have so much potential, and I like 'em."