But I do see Ben's points, there is always a risk going open source things get out of hand and break down, but thats purely down to the people who develop it.
But like you say xantier, if someone does take the code, run away and massively update it and then not share it, it has no impact, everyone else will still use the core branch and work on that one as a community, so you won't lose anyone, just that 1 guy who made his own version, and fair play to him. If he doesn't share, he doesn't share. If they did share a separate branched version, it would just be an improvement on the software, surely something Ben and everyone else here wants, and if they are sharing, I'm sure they'd happily merge it with the core branch, and suddenly Sphere is improving twice as fast.
How many plugins to web developers take, improve and adapt to their own needs and then not share back, more than I can count, but it's because they've just tailored them to their needs.
The only negative side would be if every server took SphereServer and built their own branch and did not share it back. Which is exceptionally unlikely, because:
a) I don't want to modify the core, and would assume many others don't know the C variants to do any changes, let alone how to build it and so on, some people just know Sphere script.
b) you have a load of people on these fourms, who if they did take it and make changes, would share it back.
c) everyone would find bugs and fix them in the core, rather than writing overriding scripts, they would share these back for others to have, Ego is a big thing on the internet and people would want to look like they're the best Sphere dev out there.
You would still have a core Dev team, still be able to refuse merges, and everyone owes a lot to you Ben, if it wasn't for you, 2013 would likely have been the end of Sphere.
Don't get me wrong, I'm not asking for, or begging for open source, I just think it would shake it up UO servers and give Sphere a boost. I'd always use the core builds

, RunUO was open source, and look at how many tools people made for it, and how quick new content was added when new expansions came out, not 1 guy would branch and try to add massive content like that, it would HAVE to be a team effort, there is a new expansion coming soon, and if there is 3-4 devs adding it all in to Sphere, it's going to take longer than 20 people adding little bits as and when they can.