We take adding any new UI or currency very seriously. It's almost never that the one suggested addition makes things too confusing or makes the UI too busy, but it's the continued addition of new visual elements over time. The currency rewards that fill the role of acquiring the exact card you want are gold and dust, which are both awarded via arena. Adding a pack token would increase the amount of rewards similar to gold and dust, but not at a rate I think that's worth adding new visual elements and redeemable currency to the shop and/or pack opening window. We frequently make choices that increase UI complexity, like the addition of Tavern Brawl, cards like C'Thun, or new card types like Quests. There will always be circumstances where we feel like the additional elements are worth it, this is just not one of them.

That said, we recently changed arena rewards to have less variance and are still looking for feedback on that change in addition to other suggestions.