I experienced that with the growth of application, the routes becomes very complicated. The following is a simple example

Before

# config/routes.rb
ActionController::Routing::Routes.draw do |map|
  map.resources :posts
  map.resources :comments
  map.logout '/logout', :controller => 'sessions', :action => 'destroy'
  map.login '/login', :controller => 'sessions', :action => 'create'

  map.namespace :developer do |dev|
    dev.resources :posts
    dev.resources :comments
    dev.logout '/logout', :controller => 'sessions', :action => 'destroy'
    dev.login '/login', :controller => 'sessions', :action => 'create'
  end

  map.namespace :admin do |admin|
    admin.resources :posts
    admin.resources :comments
    admin.logout '/logout', :controller => 'sessions', :action => 'destroy'
    admin.login '/login', :controller => 'sessions', :action => 'create'
  end

  map.namespace :api do |api|
    api.resources :posts
    api.resources :comments
    api.logout '/logout', :controller => 'sessions', :action => 'destroy'
    api.login '/login', :controller => 'sessions', :action => 'create'
  end
end

In practice, there are much more routes in each namespace. As you seen, there is a default namespace (without namespace), and 3 other namespaces (developer, admin and api), the routes are complicated and very easy to add or change the route in wrong namespace, as there are some similar routes in all the namespace.

After

The better solution is to split the routes into different files according namespaces.

# config/routes.rb
ActionController::Routing::Routes.draw do |map|
  map.resources :posts
  map.resources :comments
  map.logout '/logout', :controller => 'sessions', :action => 'destroy'
  map.login '/login', :controller => 'sessions', :action => 'create'
end

# config/routes/developer.rb
ActionController::Routing::Routes.draw do |map|
  map.namespace :developer do |dev|
    dev.resources :posts
    dev.resources :comments
    dev.logout '/logout', :controller => 'sessions', :action => 'destroy'
    dev.login '/login', :controller => 'sessions', :action => 'create'
  end
end

# config/routes/admin.rb
ActionController::Routing::Routes.draw do |map|
  map.namespace :admin do |admin|
    admin.resources :posts
    admin.resources :comments
    admin.logout '/logout', :controller => 'sessions', :action => 'destroy'
    admin.login '/login', :controller => 'sessions', :action => 'create'
  end
end

# config/routes/api.rb
ActionController::Routing::Routes.draw do |map|
  map.namespace :api do |api|
    api.resources :posts
    api.resources :comments
    api.logout '/logout', :controller => 'sessions', :action => 'destroy'
    api.login '/login', :controller => 'sessions', :action => 'create'
  end
end

Then you should tell rails there are 3 additional route files.

In Rails2, you can create an initializer

# config/initializers/load_custom_routes.rb
ActionController::Routing::Routes.add_configuration_file(Rails.root.join('config/routes/developer.rb'))
ActionController::Routing::Routes.add_configuration_file(Rails.root.join('config/routes/admin.rb'))
ActionController::Routing::Routes.add_configuration_file(Rails.root.join('config/routes/api.rb'))

In Rails3, you can set the configs in config/application.rb

config.paths.config.routes.concat Dir[Rails.root.join("config/routes/*.rb")]

Now routes in different namespaces are split into different files, it is easier to maintain, I only change the developer related routes in config/routes/developer.rb, without any possible to change the rotues in admin or api namespace. Besides, rails will only reload the developer routes if you only change the config/routes/developer.rb, instead of reloading the whole routes, which may be expensive when your routes are complicated.