And to add to this, it's impossible to predict the future. Even if you are privy to all relevant details to come up with an accurate "value" to attach to your shares, you have no idea what sort of deals will be made in the future (stock-backed loans or later equity rounds come to mind). All it takes is one bad-for-common deal and you're wiped out, and common is usually the first to get the short end of the stick (and the last to find out).
