Nope, tillering trees aren't necessary to make bows... but then neither are lots of other tools we use.
There are legitimate reasons to walk away from the tillering tree before the bow is finished being tillered.... like because it isn't set up to replicate the archer's holds, or isn't used properly and to its fullest capabilities, etc... such systems can't be trusted... and quite honestly shouldn't be.
On the other hand, I trust my tillering tree to a very fine degree of accuracy, better than I can possibly achieve by feel, with pictures, or mirrors and in order to make the absolute best behaving bows I'm capable of, use it right up to the very end... even come back and check it on the tree repeatedly throughout shooting it in.
It's like tuning or aligning a vehicle for peak performance by feel versus using the proper measuring equipment.
IMO, beginners, most anyone actually, will make better bows, and gain a deeper understanding of them, once they learn to set their tillering tree up and use it properly than they will trying to judge dynamic balance by feel... or by buiding to predetermined ambiguous brace height measurements.