Divinatory reasons, no. I know quite a few people who find that different decks are better suited to different subjects of divination -- this one for questions about love, that one for questions about magic -- but half a dozen is usually enough.
There are two other reasons, however, why someone might want a larger collection of tarot cards. The first is that different cards have different symbolism, and if you work with the symbols of the decks in magic, meditation, or what have you, it's helpful to have any deck whose symbolism you want to work with. Since decks go into and out of print without warning, for that matter, having a good collection guarantees that you can use a deck when you want it.
The other reason is if you're going to be teaching tarot to other people. In that case it helps if you have a copy of the deck your student is using, so you can point them to symbols and other elements of card design they can use for mnemonic and divinatory purposes.
no subject
There are two other reasons, however, why someone might want a larger collection of tarot cards. The first is that different cards have different symbolism, and if you work with the symbols of the decks in magic, meditation, or what have you, it's helpful to have any deck whose symbolism you want to work with. Since decks go into and out of print without warning, for that matter, having a good collection guarantees that you can use a deck when you want it.
The other reason is if you're going to be teaching tarot to other people. In that case it helps if you have a copy of the deck your student is using, so you can point them to symbols and other elements of card design they can use for mnemonic and divinatory purposes.