1. If you are a rough user and drop your phone a lot or get into all sorts of physically rough situations.
2. If you are an iPhone idiot (no offense here, but it beats paying for the iPhone for idiots book), you can call Apple Care's number non-stop to ask them questions about the phone.
3. If you think that you want to extend the warranty for an additional 2 years just so you are covered, that's also a reason but you can get other insurance to cover the iPhone.
If you're past your normal warranty time, you will not be able to get AppleCare. When I went in and got mine last weekend, they verified I was still under warranty and told me that if I was even 1 day outside of it there was nothing they could do.
So if it ends today, you might want to get it now.