I would like the ability to delete messages while the preview pane is open. Often I will click on a message and then start selecting messages in the list to delete only to discover that it cannot be done. Then, if you close the preview pane all the selected messages become unselected. It would be nice if the preview pane just remained open once you select a message.
** Many nice improvements in R7 beta - Thanks! **
Thanks for your message.
We already have issue #806 “Show message action buttons when selecting messages with preview window open” and issue #764 “Keep message selections when closing preview pane”. We’ll try to make these a higher priority.
Glad to hear you’re enjoying the beta! 
– Geir
When a message is displaying in the preview pane, and you have messages selected for deletion in the message list pane, only the message in the preview pane is deleted. I think this functionality should be the exact opposite. The way I think it should work is if you click delete with the active message in the pane, that message should be deleted. If you select multiple messages, then only the emails that are selected in the messages pane should be deleted. Periodically, I will be reading a messaging in the preview pane and then start ticking off unwanted messages and hit the delete button, and the active message is deleted.
This issue has been around for a while, so I thought I would bring it to the foreground.