This is an interesting point. After considering the pros and cons of hiding the Process Explorer window automatically when a process has been launched, I decided not to do that.
You can hide the Process Explorer (as any auto-hide window) by simply hitting the Esc key, or by clicking on its associated vertical tab. Both methods are very easy and fast, so I decided to keep the window open because this facilitates launching several interfaces, as you've pointed out.
Anyway, nothing stops us from adding this as an option to preferences (Hide the Process Explorer after launching a processing interface). What do you think?