I posted this elsewhere, but it applies here:
As a buyer, sniping is a great tool, ESPECIALLY the ability to "bid" on multiple items in groups and only actually bid on one. Also, as a buyer, you actual bid is not posted, so your bid does not immediately up the bidding amount, nor does it draw attention to you as a bidder, so others do not necessarily bid higher in response, knowing you want the item. It keeps the price lower.
As someone who also sells, I hate the sniper for all the same reasons. If a bidder of my item knows someone else wants it, they may up their max bid, or push the item up higher. The snipper stops that from happening.
To reply to one post here, the goal for a seller is not to simply have someone bid, but toget the most for their item, and Snipping effects the outcome.