Shell is ndeed very water resistant - in terms of resisting penetration from water - and I would also be hesitant to use a water repellant product on it.
But the impact on aesthetics is a different story, IMO. Here, shell lags behind both calf and suede. Waxy water spots and bumpy welts are often the result when shell gets wet - and they are a royal PITA to remove. I have found that a light coat of paste wax helps mitigate against this at least to a small degree, and does no harm to the shell.