If that's the case than we should always book the cheapest room on the cruise we think we will go on and worry about what we actually get later.
That only works if higher levels are available...and you are willing to pay for it.
For example, say you book 11C and lock in your price. Then later you decide you would like an oceanview and look for an 8C...but it's not available and say all oceanview rooms are soldout and the next highest is 5E...to upgrade past an inside stateroom you'd have to pay for the 5E.
Chances are you can upgrade later...but there is no guarantee that you'd be able to upgrade to what you want.