Are features optional or not in an Eclipse e4 application/plugin?

Question

I am trying to understand the concept of eclipse e4 for RCP.
After reading following the tutorial, I am stuck in the following:
Assume I create a Plugin project (no Rich Client unchecked, Actvator unchecked so no classes under /src etc)
A plugin is the smallest deployable and installable software components of Eclipse. Ok.
Now I can create a Product Configuration which is based on Plugins or Features.
So the Product Configuration delivers the plugin etc and if I understand correctly the Feature is optional. Assuming that I am correct (don't know, am I?) I add to the product configuration the dependencies:
org.eclipse.emf.common and org.eclipse.emf.ecore and press Add Required Plugins
My product confuration is selected to be plugin based.
What I don't understand at this point, is that if I don't define as part of the dependencies my plugin, I get a missing plugin when I run the application (I have checked the validate plugins before run). But why? The plugin, has already been defined as a "Definining Plugin" in product definition (New). Why do I have to explicitely add it also here?

Anyway passing that, when I try to launch a window (yes I have created a New Application Model and defined a Trimmed Window) I get runtime exception

java.lang.RuntimeException: Application "org.eclipse.e4.ui.workbench.swt.E4Application" could not be found in the registry. The applications available are: org.eclipse.equinox.app.error.
    at org.eclipse.equinox.internal.app.EclipseAppContainer.startDefaultApp(EclipseAppContainer.java:248)
    at org.eclipse.equinox.internal.app.MainApplicationLauncher.run(MainApplicationLauncher.java:29)

I can only assume that this happens because a plugin is missing which is: org.eclipse.e4.ui.workbench.swt (I got this due to enabling validation of plugins)

Why do I get this exception? I believe that I get it because I have not added the org.eclipse.e4.rcp as dependency.
But I can not find it as an option. Turns out that this is available to be selected only if the product configuration is based on Features. It is not available for Plugins.
Additionally the same process but defining a Feature and "attach" it as dependency to the product configuration works! I.e. no runtime exception and the window shows up.

So what is happening? Are features mandatory? I don't understand. I thought they are optional. Could anyone help me figure this out?

Update:
If I manually add org.eclipse.e4.ui.workbench.swt as dependency (and press add required) in the product I get:

!ENTRY org.eclipse.osgi 2 0 2012-08-20 18:36:52.044
!MESSAGE One or more bundles are not resolved because the following root constraints are not resolved:
!SUBENTRY 1 org.eclipse.osgi 2 0 2012-08-20 18:36:52.044
!MESSAGE Bundle initial@reference:file:plugins/org.eclipse.e4.ui.workbench.swt_0.10.1.v20120525-2014.jar was not resolved.
!SUBENTRY 2 org.eclipse.e4.ui.workbench.swt 2 0 2012-08-20 18:36:52.044
!MESSAGE Missing required bundle org.eclipse.core.databinding_[1.2.0,2.0.0).

Answer 1

If you are creating a plugin based product on Eclipse 4, you still need most of the plugins that comprise the org.eclipse.e4.rcp feature. Try adding them from the list in http://git.eclipse.org/c/platform/eclipse.platform.ui.git/tree/features/org.eclipse.e4.rcp/feature.xml