I understand your point, but don't agree fully.
Adding videos to youtube account is where creating a custom thumbnail can be done... if you choose to.
Sometimes, this is not an issue for people.
The bigger issue is that with Gallery, you are forced to create a thumbnail or you can't add a video. This is not practical for users who are looking for turn-key gallery solutions. You need to keep in mind that many of us create websites for our end users - many don't either know how to do the image preparation work in order to add a video, or simply don't want to do this step.
Is there not a way you can allow videos to be directly added with just the embed step, and then, if desired, an option to add a custom thumbnail (as it works now)?
You are building a platform (Gridbox) that is changing the way Joomla sites are built - simple and direct editing.... please keep this in mind when updating your components like Gallery.