There's nothing cruel about an undisclosed reserve price, which is a standard practice in non-online auctions.
One good reason to use an undisclosed reserve price is if you really have no idea of the fair market value of the item. Let's say I want to sell a Canon Filter Previewer (a non-existent accessory about which I once had an incredibly vivid dream) and since I can't find any records of previous sales, I have no idea of what somebody might pay. $1? $10? $100? $1000?
Let's suppose I figure it must be incredibly rare, and decide that if I can't get at least $500 for it, I'd rather just keep it. Now, if I simply listed it with a $500 minimum bid, it's possible I might get no bids for it at all. That would leave me with no idea whatsoever of what people might have paid, so I'd be getting no useful information at all in return for the cost of listing my item.
On the other hand, suppose I list it with a minimum bid of, say, $50 and an undisclosed reserve of $500. Now I can find out something about the item's real market value, without the risk of having to sell it for less than my target price. If I get active bidding topping out at, say, around $200, I have a notion of what I'd be likely to be able to get if I relisted it. Then I can make an informed decision about whether to keep it or sell it.
I agree that disclosing the reserve price is just a waste of your reserve fee. If you know that you don't want to sell the item for less than X dollars, just make X dollars your minimum bid.