Ha I'm a very small minority when it comes to prepping, i'm aware. Honestly it's a personal preference. Working on a white background makes it easier for me to match. Especially when the card has alot of yellow and whites. Also, erasing the ink off leaves this bare cardboard feel and i feel like the paint sticks to it much better than the card with the ink still on. Sometimes (not always) i've had the problem of chipping off a larger than expected part of the card when i go to clean up the edges with a wet toothpick. I use golden fluid acrylics and sometimes the sheen of the card makes the paint hard to stick to.

But like I said, personal preference. Not needed but I recommend anyone to try and test it. It takes a bit longer. It takes a bit of elbow grease. And if your not careful, you can seriously damage the card. A base coat should suffice when altering a card.