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.