Why would I want to use unicorn or thin instead of WEBrick for development purposes?

It is important to develop as closely as possible to the production environment. It helps ensure that an application will work as expected when deployed into production, instead of stumbling upon bugs at runtime.

This issue is alleviated with the use of Continuous Testing on a Build server that replicates the production environment. Even though you are not actively developing on an identical environment, the Continuous Testing gives you coverage that the application is functioning in the expected way.

As to speed, the performance hit running a Rails app in development mode will negate any benefit the various web servers brings.

In addition to the other answers giving a pretty good overview already, there is also a technical reason you might want to consider using unicorn over WEBrick:

WEBrick does not support subdomains. Support for HTTPS is rather hacky to implement.

So if you have an SaaS application using subdomains, or if you simply want to have admin/api/... subdomain, then WEBrick is not an option. There is POW for Mac OS X, but this won't work for Linux developers.

My personal experience is that Unicorn is much much faster then WEBrick when using a remote machine (ubuntu, 4 cores, 8G mem, connecting though VPN -> ssh) as your development environment (as I do) -- I see sub-1 second page load times with Unicorn, while WEBrick takes 3 to 5 second or more. I'm not sure why, it may have more to do with my network then anything else, but that is what I personally see.

I haven't used Thin for development, as I've read that it requires an additional gem to allow for class reloading in development mode (I can't personally validate the accuracy of that). Also, I am more familiar with Unicorn and use it in production.