Please sir, I don't agree.
When you see an item selling for $10, if NOBODY else bids (not even the current high bidder) then it will sell for $10.
If anybody else bids (to be clear this is you we're talking about, not a third bidder) more than an increment above $10 then the price will change, but there is no guarantee than it will be $10.50.
The current high bidder may be extremely keen on the item and may have bid $100. then when you bid at $20 the price moves to 20.50 and he's still winning - and when your snipe hits at $40 the price moves to $41 and he's still winning.
The current winning amount gives no indication how high the current winner has actually bid. Except in the special case when he has bid less than an increment above the previous high bidder but more than an increment above the previous winning price in which case the bid history and current price reveals his actual bid.
So when you bid 20 and follow up with a snipe of 40 even if nobody else bids the outcome is determined by the other bidder's maximum.
However if the winning bidder has only bid $15 dollars then your bid of $20 may make them wake up and bid again - they might just bid $16 to test it - or if they aren't very familiar with Ebay they could end up eating away at your bid of $20 until they get the item at $20.50 so when your snipe of $40 hits you win at $21 - so the manual bid of $20 ended up costing you $5 because you woke the other guy up.
Or worse perhaps they wake up properly - they pay attention - see your bid winning at $16 - look at your bidding history and see that you regularly snipe over your own bids and they slap in a serious bid of $53.77 and they win for $41.
You can try to guess what is going on in other people's heads - the benefit of sniping is avoiding all that - look at the item decide on the price you want to pay (perhaps add just over half an increment or even an increment and a half sometimes - set up the snipe and walk away. After the auction you've either won it or you haven't, but if you haven't it was because somebody actually bid (whether they sniped or not is unimportant) more then you were prepared to pay and they did it completely independently of your bid.
Except in some very specialist areas there are few enough collectors that we know how the other collectors behave - and half our battle is working out who and how many other serious collectors have seen the obscure and badly listed rarity we're looking at sniping.
The reason I add a bit is that I really hate looking back after an action I've lost by a small margin and saying "I would have paid that much" - if I really only want to pay $50 then sniping at 51.77 pretty much guarantees that the minimum it will sell to somebody else is 50.78 and will probably be 52.77. So I don't feel so bad - But if I still do then my $50 was wrong and I should have been working off a basic price of $60, or whatever, before I add my little bit.
John.
John SL
