For me, it's ease of use. I'm not the primary administrator, but 6.0 and the included web client provides me all of the resources I need to do my job without headaches. I'm the company DB/BI manager so I have a small subset of VMs that I manage, but I can't remember a time where I've had to spend any time on a forum or with support because vCenter wasn't performing.
I see where they're going with the web client, but I still prefer the thick client. While all of the feature set I need is in the web client, it just feels slower than the thick client. The primary admins of our VMWare environment dislike the web client enough to not upgrade to 6.5.
If you have more than 1 or 2 hosts, it's worth it to get vCenter so you can get the high availability and time savings it provides. Even if you haven't considered it in quite some time, check out the current version...it might be time to re-evaluate.
We have 8 hosts to manage. We needed a single interface to go where we could manage our infrastructure and provide high availability. The benefits I've seen are that I can do everything I need to do without surfing through 8 hosts and having to schedule downtime of my SQL servers when infrastructure repairs need to be made.