Well, I'll tell you one thing, Adobe has a pro-rated upgrade policy and if you wait too long between upgrades, then you put yourself out of range for upgrades and your only solution is to purchase a whole new version. I've been using AdobeCS and never upgraded to CS 2, 3, 4, or 5 and now I'm not qualified for an upgrade. Adobe is running a small 15% discount that ends today however. You may need a direct link to that upgrade so if you don't find it on their website, contact me and I'll try to send it to you if anyone is interested.
Why would I want to upgrade at this time? Well, as explained above, if you don't keep your upgrades relatively current then it costs you more money in the end. Also, I have some difficulty with accepting customers files that are created with newer version. Also, I have plug-in software that only works with newer versions. Also, there are some newer features of the latest versions of software that you might like to take advantage of. I think it's the unfortunate cost of doing business to keep all your software current.