Please, find my answers below to your questions:
In rtMedia it seems that no message is attached with the image upload. Instead, you'd have to manually edit the image to type a message or upload from a different location (maybe... haven't checked).
I guess you want the BuddyPress activity post content to be attached to the media you are posting with the same activity. I would like to clarify that BuddyPress post content and rtMedia media description both are different things.
BuddyPress post content is specific to that feed post. It is not considered as the media description. If you want to attach the media description from activity post then you can add it while uploading media the same way as you do in the profile media.
Here is the screenshot:
Is there a way I can get this functionality or is there logic behind why it's not handled this way? Or maybe a feature request?
I also found that in the little image thumbnail is where you edit the title and description to the image. Kinda hidden and overly complicated... still makes since to me that rather then having it that way have it display whatever is written in the post. As far as the "title"... really isn't important on my site.
There is not any functionality available to bind post content with media description for now.
However, you can try to achieve this using some custom code by yourself. But first, you will need to define your requirement clearly by checking multiple scenarios.
For example, try attaching multiple media with same post update. In this case, will you prefer attaching same text to each and every media attached to the post? FB do not attach the text to media if you upload multiple media.
Please, let me know if you have any doubts.