No, it's not that the NCAA relies on the RPI. It's that the process allows it to defend just about any result.
There are numerous problems with the NCAA Tournament's selection process -- namely, that CBS turns what could be a ten-minute show to reveal the brackets into a two-hour extravaganza of commercials and Charles Barkley attempting to use technology -- and Glenn Logan at A Sea of Blue touches on some of that in a great piece, but most people who complain about the process, about the teams that were snubbed, about the teams that were included, or about the RPI are missing a far more basic point about the selection process.