I am working on a plugin for this exact purpose. render_component does suck hard and is way slow because it has to instantiate a whole new request/response and controller object. But I do think there is a need for a light weight way of making small ajax applets or page parts that can get rendered in a view in a similar way to render_component without all the baggage but also need to be able to handle ajax callbackls in the controller. So I have a plugin called 'cells' that aims to solve this problem. I haven't released it anywhere yet but you can grab it form svn and see what you think.
It works by piggybacking off of ActionController::Base or ActionView::Base depending on whether its handling a request or whether it is being rendered within another view sort of like a component. Keep in mind that this plugin is not about sharable betweek apps reusable high level components that everyone hates. Cells are about encapsualting small page parts and giving them a way to remain independent from other controllers. Using this technique you can accomplish something akin to render_component in a view, only its around 21 times faster It also makes you add only one ActionController c lass to you app. This controller is a dispatcher for all your cell controllers. Each cell is a small MVC stack that lives in its own folder. So in your app you add a directory called RAILS_ROOT/app/cells. And in cells/ you can have your cell's views, models and mini controller.
Here is the svn url. It is a full rails app with a small todolist sample 'cell' included so you can see how it works. PLay with it and lt me know how it works for you. When i get some more time I will make a it a more genrally available plugin. I am getting a lot of good use out of it and I think it solves a problem that a lot of folk struggle to do in a clean way with rails.
svn co http://svn.devjavu.com/cells