I bought either a $10 and a $5 or two $5s (I really can't recall, though I think it was a $10 and a $5) when I first started playing the game (about two weeks after it went live) because I raced a lot and really wanted a souped-up car of some sort because I really needed it for racing. I think this was even before the $500,000 "stimulus" but I could be wrong (damn, that seems like years ago).
This was ages before the money glitch gave me $26 billion, and I spent most of my free time in the game there toward the end frantically modding cars for newer players who missed the glitch to sell or keep 'cause I wanted to spread the wealth around.
We were all low-level at the start of the game and no one was grinding Rooftop Rumble 24-7 - our "grind" was the Boneyard survival, and while it pays well, it does take a lot of time.
Now that everyone grinds either Rooftop or Coveted, buying a card seems like a waste of money to me now. I wouldn't buy another one. I'm sure they were a waste of money back then, too, but I felt like I had less options, if that makes any sense.
As far as people buying them, hey, it's your thang. Do what you wanna do. I can't tell ya who to sock it to.