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I understand the appeal and I was looking at getting one also when I first started sidemount but now I would absolutely not want to use that thing. The only benefit from it would be so that your tanks even out and you can breathe off one regulator during the dive. It's just more points of failure and possible entanglements though. I'm assuming there must be a quick disconnect for your hoses and they aren't fixed connections. Even if you had a quick disconnect it could allow water in the line if you had to connect or disconnect anything.
I want to be able to remove my tank and hand it to my buddy if he needs it rather than sharing air to safety. I also want to be able to tank my tanks off at any time with out anything else making it more difficult. If you did have to switch tanks with someone and there's a quick disconnect then it will allow a small amount of water into the system that will then go through your second stage, which isn't a good thing.
Like I said, the initial thought of it sounded great until I started diving sidemount more. Now I don't even think about switching my regulators to keep things balanced and I just do it naturally and effortlessly.
I agree though, to each there own so if it works for you and you like it then go for it and I'm happy for you. Are you going into caves or wrecks using that manifold or open water?