Wouldn't it be possible to extract the getting viewId logic from MainPortlet.render method to separate protected method To be more precisely, this code:

        // Get the inital view that is configured in the portlet.xml file
        PortletMode portletMode = renderRequest.getPortletMode();
        String viewId = null;
        if (portletMode == PortletMode.VIEW) {
            viewId = portletConfig.getInitParameter(Constants.VIEW_KEY);
            if (viewId == null) {
                if (log.isErrorEnabled()) {
                    log.error(Constants.VIEW_KEY + " is not properly configured");
                }
                throw new PortletException(Constants.VIEW_KEY + " is not properly configured");
            }
        } else if (portletMode == PortletMode.EDIT) {
            viewId = portletConfig.getInitParameter(Constants.EDIT_KEY);
            if (viewId == null) {
                if (log.isErrorEnabled()) {
                    log.error(Constants.EDIT_KEY + " is not properly configured");
                }
                throw new PortletException(Constants.EDIT_KEY + " is not properly configured");
            }
        } else if (portletMode == PortletMode.HELP) {
            viewId = portletConfig.getInitParameter(Constants.HELP_KEY);
            if (viewId == null) {
                if (log.isErrorEnabled()) {
                    log.error(Constants.HELP_KEY + " is not properly configured");
                }
                throw new PortletException(Constants.HELP_KEY + " is not properly configured");
            }
        }

It would be useful to have an ability to override this logic in the subclasses. Personally, I'd like to add an ability to show some view based on request parameters.