Honestly, those reasons are solid but I think the community can play a huge role in the reasons that you mentioned.
If you release Dominion as an "unofficial" release people will go in with a different mindset, knowing everything isn't perfect. Most of the main imbalances that we've seen have flown over playtesting's head that the community ended up discovering (see: Triforce Irelia), and a lot of balancing that is done in SR is actually found through community conversation (not just whinewhine qq op, but legitimate debates as well).
Another things it that the community is very nice about spotting things like bugs, I'd be willing to be 90% of the bugs that Riot has ended up finding and fixing was due to community reports.
I can understand optimization too but an "unofficial" release or a "beta test" for the map would explain that this isn't the final product, it's just the community is being a part of it's construction.
tl;dr, I want ****ing Dominion. Now. Please?