Not sure why he recommended calf but he should know. I do know that when shell gets wet, the leather has "spots" on it when it first dries out. Maybe he was thinking about a leather that you could apply waterproofing to.
Well, here's what he said (seems like a nice guy, I don't think he'd mind my posting this).
"Both leathers will hold up physically. That is, the leather won't give way or anything like that. But water can affect the finish of shell cordovan shoes, making the finish rough, or causing welts, which can be difficult to entirely remove.
For that reason, the Alden Shoe Company is usually hesitant, and so is our shop, to recommend shell cordovan shoes for use in wet weather, or as rainy day shoes.
For harsh weather conditions, we will normally recommend a shoe other than shell cordovan, since the customer would probably prefer to keep the finish of his shell cordovan shoes nice and smooth and not risk damaging the finish by purposely wearing the shoes as rainy day shoes.
That being said, I'm sure lots of men in New York for example, probably wear their shell cordovan shoes in winter, and don't have many problems. "