Actually, I am going to disagree with that statement. Your snowboard helmet would actually offer more protection than many of the foam based climbing helmets. But, would you want to wear your snowboard helmet while climbing rock in the summer, I'd be inclined to say no. It would be so hot as to be very uncomfortable, it's designed for maximum coverage and insulation.
Here is a general statement, that can freely be attacked, but many helmets can server double duty. Though you must weigh the advantages/disadvantages of doing so. Example, I've seen people use bike helmets for climbing, while they are better than nothing, they offer little protection to the sides of your dome and are designed more for a single major impact on top. Yet another example, I ski in my mountaineering helmet on occasion, breathability is great for climbing, but it doesn't offer the same amount of impact protection as my ski helmet. So you have to think about what the helmet was initially designed for, and decide what you are sacrificing using it for double duty.
Thanks Eric for putting me on the right track. I guess I was miss informed about helmets. Damn salesman! I have three helmets and should only have one motorcycle helmet!