Well, I'm sure you see why your idea wouldn't work. Someone like EMC will come along and quote raw capacity, without any overhead for Snapshots, or the filesystem, without hot spares, etc. And they will come out look better than NTAP. Heck, they ALREADY try to compare non-parity protected systems with NTAP's parity protected systems. It doesn't matter if the customer gets it, flips the switch, and says, "Hey, where's all that space?" It's too late; the sale has been made, and the company is already invested in EMC. The switching cost to another vendor is non-zero.
Another thing to remember is that since EMC and other vendors are more "established" names, they are often purchased as part of a higher-level strategic decision, by upper-level managers. The mid-level, detail-oriented systems administator doesn't have much choice, even if they understand the distinctiong between the methods for measuring available disk space. And if that admin *is* able to influence the purchase decision, then he is probably smart enough, like you, to know what's what when it comes to capacity and can see past the marketing layer.
Bruce