I also upgraded from a Titanium AW Series 5 to a Titanium AW Series 6 - and I will probably upgrade again next year.
To me it is worth it, but I don't need a justification, I just want to.
Me (and my family, wife and daughter) are in the lucky situation (and I really mean it, I feel grateful for it) that we can afford these kinds of things. I just love Apple tech and I love owning the latest & greatest.
Maybe this stems from my childhood days, when I grew up in modest circumstances and we simply couldn't afford many things. But through hard work and persistency, I am now in a situation, where 800 or 1000 Euros for an AW every year don't make that much of a difference.
The old Watches (or other Apple products for that matter) are either traded in with Apple or passed on to members of the extended family (whom I charge the price, a potential Apple trade-in would have earned me, if they insist on paying me something). I do not want to go through the hassle of selling it and dealing with all kinds of weird buyers out there.
As a matter of fact, the AW Series 6 provides me with a (in my opinion) noticeably faster user experience, somewhat longer battery life, the oxygen sensor, improved heart rate measurement (since I am much into fitness and sports) and a slightly better AOD. To me worthwhile, if incremental, updates.
So as long as you can afford it an feel like it, why not upgrade? Any reason is a good reason. Even no reason at all.