What's really bugging me right now is that my brain is set in oldest mode and I keep thinking answers are older than they are. I'd hate for my vote to accidentally go to an undeserving dupe answer posted 10 minutes later as a result of this. Sure, I might get used to it but I'd much rather see oldest return.
It makes more sense to replace newest with active, IMO. Before active came along, newest was mostly useful to a question asker to see the latest replies to his question. active takes this one step further, sorting answers by their posted or, if applicable recently edited timestamps. This makes the newest sort order fairly redundant — partially because newest provides no additional benefit over active, but especially in the case where none of the answers have been edited because they will be sorted exactly the same for both active and newest.
oldest is useful to both voters and askers. In an ideal world, an asker would accept the first posted "best answer" where there a few are identical. For voters, you have 30 votes to spend per day and a lot of high-activity users will cast those votes sparingly. On a very easy question in a popular tag, 5 or 6 similar answers can be posted within 1 minute, you don't want to waste 1/6th of your daily votes up voting each correct answer. It makes sense to up vote the first "best answer". With active, the only way to do this is to manually check the timestamps or sort by newest and read from bottom up - which can be a pain if there are multiple pages of answers.