Technically, whatever you are saying is all good and doable. Further, if you ask me, I would also recommend doing the Promotional Content as multimedia content with text in the metadata. However, if I place myself in the shoes of Marketing people (who will actually be owning the promotional content and possibly the non-technical people), from Inline Editing as well as from the editing in CME point of view, it might be a big no from their perspective. If the Content Authors are somewhat technical (even minor affection to technology is sufficient), I can't visualize any problem with the approach you are suggesting.
Further Update: Also, if we talk about metadata, it gives us a concept of data about data and many of us will think a metadata we will be using more often for back-end work/logic and not for displaying on the webpage. Few CMS professionals have preference or rather thumb rule - whatever is visible on the site, should be part of actual content, and whatever is not visible and just describe a content (or help as a supporting content of a content) should be part of the metadata.