it is possible to shoot without one - if you are VERY familiar with your stuff - i know of one top commercial photog that doesn't use one - but that is because he has been using one for the past twenty years shooting film and he KNOWS his lighting ratios and what kind of f-stop to expect from his lights with diff light modifiers and at what distance and at what power level. Shooting digitally just "confirms" with what he knows. He can get his lighting set up done (e.g. 3 lights) within the first 3-4 test exposure.
But that is him.
for myself, if i am not critical abt the output - then it will not be essential - guestimation is enuf.
but if i want to operate at the top level, then it becomes essential.
And for photogs interested in learning abt lighting ratios - it is essential.