I despise the fact that we've taken a precious tool of our trade and reduced it to nothing more than a collectible baseball card.
I despise the fact that we've got magicians practically wetting themselves over the next new deck release from their favorite company when all decks generally function the same way.
We have taken an essential item and reduced it to nothing more than a trendy commodity. We've taken the focus off the magic, off the experience the spectator gets, and instead placed it upon the deck of cards themselves. We've exchanged the value of entertainment for the value of bragging rights.
Are they essential? I think 'common' would be the better word here. Playing cards are not essential to a magic performance. You can easily have one without them.
Personally, I look at custom cards the way I look at other accessories. I wear a certain coat, a certain pair of shoes, certain bracelets, etc. Or, for example, a top hat with a skull on it. They are all subtle expressions and convincers of your character. Congruent imagery enforces the character. Let's take a slightly different example - Two types of tarot readers, who do you think would appear more genuine? The one with a wooden 'gypsy caravan', piled with cushions, lanterns, brass tea pots, wearing traditional gypsy clothes, slightly worn from travel, and diving the future with a hand-painted deck of Tarot cards which have clearly been around the world ... or someone with that same deck, but wearing jeans, a blouse and make up, stepping out of a mini-van? Cards are nothing more than an accessory and having a design that fits your character goes that much further to making the whole image work.
Now, some people do seem to believe that all they need is a shiny deck of cards and not the skill to use them. But that's always been the case. "If I buy this Azra, I'll be the best magician ever!" Or how about magicians who have collections of various types of Cups and Balls? Or Hippity Hop Rabbits?
Magicians are prone to obsession. If we weren't, we wouldn't be able to get very good at what we do. With that is the tendency to collect things. Right now the popular thing is cards. I admit, I am a bit of a collector myself though I don't go crazy about it. - Of course, I've been mildly obsessed with cards since I was a kid and figured out how to stack the deck playing Uno against my neighbor's granddaughter.
How is this reducing the value of the cards? If anything, they are given more value. They have been taken from something considered to be a throw-away consumable prop to something to be collected and displayed. The value has gone up.
Also - despise is a really strong word. Do you really despise this? Is it bothering you that much?