I think that reversed cards can be important but are not necessary. However, what if all the cards are reversed - one should wonder if their entire deck is actually upside down to start with?
I think if one knows their deck and their cards, it's not about being literal about reversed or upright as reading from a book would interpret. I think that the intuition steps in and interprets the meaning of a card regardless of how it is placed. Also considering the second card of a celtic spread, it is sideways, and different readers may interpret it upright or reversed depending on how it is laid and their personal methods. I think if a card is reversed and the literal meaning read out of a book (which is often the case) is the only thing considered rather than considering the card as a whole then the reader may be missing the point of working with the tarot. Reversed cards do not mean explicitly different meanings than upright, the reversed meaning relates to the upright meaning of the card. The card is a whole.
I think that if someone is in the early stages of learning about doing tarot readings then starting with upright is the way to go, which will allow one to learn the meanings of the cards in general and have a good understanding of the tarot. Then when the upright is understood as a whole, then the reversed can be applied and understood as relating in many aspects. I think if someone is doing readings for someone else, and reads literal interpretations of a reversed card from a book, then they may want to learn the tarot in the upright first, especially if doing readings for others and especially if getting paid.
I think upright has a lot to offer on their own. 78 cards intermingling, providing meaning upon their relationships, not as individual, separate interpretations, provides for many more possibilities of consideration that a lottery could dream.
A lot of people think that reversed cards are "negative" which is not the case, many cards in their upright could be interpreted as negative, and of course it is all dependent on their relationships.
It has a lot to do with shuffling as well. A lot of people don't intentionally mix their cards so that there are upright and reversed throughout the deck on purpose. If one intentionally shuffles their deck to ensure there are reversed, I think it takes away from the natural progression of card relationships through synchronicity.
I know a lot of people don't even have a grasp of the upright positions without looking at a book, after all the tarot is complex and takes a lot of study to understand, and can be studied for decades even. I think reversed should be understood as one learns, but upright should be the focus when learning, with the reverse as a compliment to the understanding of the card as a whole. I couldn't imagine someone even considering getting paid for doing readings if the reversed meanings are literal and from a book if upright on their own haven't even been grasped at least at their minimum without the requirement of a book yet and that usually takes most people dedication to study for a long time. I think if someone is well versed in the tarot, without the requirement for book references, then reversals could be included, and understood as part of the whole. I think a solid foundation of upright should be the step prior to inclusion of reversals. It depends on where the reader is in their experience and abilities.