While I see your point, this is a type of randomness I wish D&D would embrace more.
The normal model for randomness in D&D is that you have a predetermined array of options, you pick one, and then the result is determined randomly: "Okay, I have chosen to attack with my longsword, now I roll to see if I hit and how much damage I do."
In contrast, Magic: the Gathering provides you with options selected randomly--these are the cards in your hand. The effect of a card is 100% deterministic*. By putting the random element ahead of the decision-making, it prevents "pick my best option and spam it" (a common problem in D&D across all editions) while increasing the player's sense of agency (since you will never have your chosen option nullified by the dice).
With this form of spellcasting, your chosen option remains as reliable as before: You pick a spell from the ones you have left, and it works just like always. But you don't know if you will have that same option available next round.
*Okay, not quite 100%. There are a few cards that call for die rolls or coin flips. Also, many cards let you draw more cards, which straddles the line--this option's deterministic effect is to give you more randomly chosen options.