I will talk about a very short 5 to 10 min extension at the very end of a fixed-ending auction..
For example, an auction that ends exactly at say Jan 19th at 3:00pm pst.
For me, no short extension at the end makes it great for the person who has not bid yet since they can just wait for that exact time and Bam, bid a dollar more than the high bidder in the last few seconds an dthen win it.
No short extension is bad for the current high bidder though, since they can't use a proxy on themselves. For instance, if I am high bidder with $10 bid, if I am worried someone will snipe me with 5 seconds left I can't place a $30 proxy or something like that... that's just bidding over myself and making myself pay more if there are no snipe bids. But if there is an extension of even one minute (if say a bid is placed within 5 minutes) then the previous high bidder has a chance to re-bid (which is also good for the seller

)
Again just talking about an extension that just delays the auction for 5 or 10 minutes if a bid is placed at the very end and not what was in the poll.