I think what Allonym meant is this: IF you start with both The Tower and another Tarot in hand, you can either:
1. Play the Tarot for free. But you'll have to discard it if/when you decide to play The Tower.
2. Don't play the Tarot for free. Play The Tower first, and the Tarot later to discard the Tower, paying all costs.
It's an interesting decision, but if you already have The Tower as your basic weakness, you might decide that you'd just skip over all the Tarots so you don't even have to make this decision.