As a longtime attendee and panelist at BronyCon's past, I have to say that I really don't like this concept either.
I understand THE IDEA behind it...but where I start to have issues is when one really thinks about it. In-theory, the idea of accepting "other content" into the convention is to allow other people from outside the Pony fandom into the spectrum.
Here's the problem: Otakon is an hour away, and is the same weekend. It wouldn't make sense to try and sway people to a Pony convention by simply offering an opening for non-Pony stuff, especially when a far more "wide-open" convention is happening an hour away...
Don't get me wrong, the idea makes sense from a general standpoint. However, TrotCon tried this before, and it DID NOT work out well, which is why they dropped that idea soon after. I think this may go the same way...
I think this is a knee-jerk reaction to last year's attendance drop, but I think it's fairly understandable that the date-change hurt it. I wouldn't have announced such a concept like this until next year, as I think this year's BC is really going to be a determining factor in where this convention heads in the next few years.
I'm fine with them trying something new, but as many before me have stated, I'd really hate for this to ruin the overall togetherness and sense of community that BronyCon seemed to have as an "all-Pony convention".
And before I end this post, let me make this clear: I still love BronyCon and everybody who runs it. You guys have run a multitude of conventions that give me something to look forward to and you've changed my life at numerous points with these conventions.
I post all of this above out of love, I promise