Thin clients are really incredible. But just like anything, there is a point at which it becomes beneficial. Thin clients require a fairly huge investment in infrastructure. You are essentially streaming a desktop environment over a network - so you need to ensure a level of network quality (QoS) to make sure that the system is stable - specifically video and audio.
I've worked a bit with Citrix and have seen how well developed this technology is. For a corporate environment, it makes a lot of sense - especially in terms of data security and retention policies. Everything is controlled on the back-end and no data is stored on the client. So laptop theft becomes an issue of the past.
If you have the right number of clients, it certainly is cost effective. Microsoft offers a different license for thin clients (less expensive... but if you're large enough to use thin clients you probably have a site license anyway). One of the biggest benefits is cutting down on helpdesk resources (specifically people). You'll cut down on the number of times you'll need to send a tech to a workstation to install software, upgrade hardware, etc. All of that is done on the server side.
I could go on and and on - but I think you get the idea that I really support thin clients. You just have to understand the point at which it pays to invest in the infrastructure. It's costly to set up and maintain, so it would be silly to implement for 10 users.