Yep, Yahoo has that feature. I would know, being a bid sniper myself.
Why do it? Well, it's a way to get stuff cheaper. Say you and "Guy A" want a certain item. "Guy A" bids a max of $20. You know if you bid higher than that and he sees you, he'll bid higher as well (since that's how good the item is priced.) Your best bet would be to wait until the next to last second, and then bid your max. Either you get it at what you would pay for it... or you don't, and "Guy A" pays more than what you think it's worth.
The only way you would dislike snipers is if you're a buyer and have the "Guy A" mentality.
It probably doesn't need to be said, but I hate the "add on extra time" feature.
Also, Don't trust Yahoo auctions. I've almost been ripped off quite a few times there, but luckily caught them (sometimes just before I had to pay.) Not sure if they're still that bad though...