Like mammikoura said, they'll have extra features, but as long as you have definitions coming through, you'll have support.
Newer versions are for people who are just about to buy one probably, as it'd be a bit weird if you can imagine it. These people are about to buy an antivirus kit, and see 2001 up on stock. It'd mean them having to update, etc, whereas, most would rather see a new version, maybe with a few features thrown in, and not have to download as much defintions. Especially for people still using Dial-up.