Open is better in this case.
Why? Well, aside from the fact that it means the standard can be better understood, instead of relying on the hope that the non-open one will work right, Flash is also a ***plugin***. The #1 problem I run into with Firefox, and the fact that I prefer to leave it open is.... flash movies. They eat up memory, then don't release it, and eventually that lags other applications and/or prevents Firefox from closing properly, when I do shut it down. Some clients, which **already** have huge memory footprints, like say.. the various Second Life/OpenSim clients, do not support **any** plugins at all, just HTML, CCS, and other standard features of those "inbuilt" non-plugin systems. I am sure other people can think of cases where plugins are either not available, not a good solution, cause problems, or are simply not a viable solution.
What HTML5 gives everyone is a way to include video without all the external, third party, junk slapped on top, much of which doesn't, and can't, integrate gracefully with what ever you glue it to. For me, at least. Flash is a ***major*** road block to dozens of things I would like to do in-world in Second Life, including running free TV streams, especially when many people already have issues running the Quicktime streams, which are supported, in the client, without it crashing on them as well, never mind adding yet one more, and from my experience, less stable, one. It makes about as much sense to me as the discovery that Yahoo refuses to update to 5.x PHP, or provide archival tools in their small business web service, because, "At this time we don't think people need that." o.O People don't need a faster way to archive and download/upload large batches of files from their pages, **at least**? Give me a break.
Same mentality. Everyone uses X, so lets support X, even if Y would make more damn sense, and be usable by more people. Bloody annoying.