I don't like it, but it may be the only way.
While I (admittedly) have not had the time to be engaged with this site, as a long-time recording engineer and a filmmaker, I would argue that the overlap in technical knowledge and ability to give answers between the two communities is largely a one-way street.
I work with many filmmakers who are only filmmakers. They come to me with audio questions all the time because they know I'm "an audio guy". I am capable of answering questions which other filmmakers are not capable of.
That's less true of the opposite. Audio engineers by-and-large are not going around trying to figure out how to do something in Final Cut, or which camera to buy unless they are coincidentally also working in that industry.
Audio is (in this day and age) inherently a part of film. Film is not inherently a part of audio, though.
The problem faced is that both communities are too small and insufficiently engaged in these sites to exist on their own. Putting them together forms a larger community, but that community may not have any coherence. It may even cause a greater number of unanswered questions, low-quality answers, or misinformed/non-expert answers which will only lead to a failure of two sites instead of one.
Long story short, I think this proposal provides a lot of benefit to the Video and Film community, and dilutes the Audio community. However, if it's the only way the site can survive then so be it.