I've thought about the same thing, Riverhead, but I think the essential issue is the reserve BTS is sort of "owned" by all the BTS holders (as they can vote to have it used to their benefit). So this is similar to when a company has purchased shares of it's own stock. In such a case, I think the valuation of the stock held by the company is implicitly priced into the value of the "regular" shares held by the shareholders.
In the case of ripple, I think the shares in question were held by founders, not by all the coin holders. In that case I think it's fair to count it as part of the supply (although maintaining a supply of your coin that's not generally known to the general public seems quite underhanded, if that's what happened). But I never paid close attention to what ripple was doing, so I'm not accusing anyone there of anything.