Re: I've been pointing this out for years.
Ummm...you really don't understand how this works.
Take the Nestle Fairtrade chocolate scheme. They don't make all their products Fairtrade, just some of them, but they don't have entirely separate production lines (You'd need to for Organic, but that's different).
So, they say "We buy 1.2 million tons of cocoa each year. We need 200,000 tons to make the Fairtrade products, so we make sure that each year we buy 1 million tons of standard cocoa and 200,000 tons of Fairtrade cocoa". They get mixed up, but the fact you're buying Fairtrade has an effect on their practices, so it has the positive result you want.
It's not an exact analogy because cocoa can vary in terms of nutritional value, whereas electricity is exactly the same no matter where it comes from. But what you're doing is influencing buying practices. If you are on a 100% renewable scheme and use 1000 kWh, then it guarantees that 1000 kWh of green power is put into the system instead of 1000 kWh of non-green.
The whole "separate power lines" argument is a fallacy - otherwise every time you switched provider they'd need to lay new cables to your house. All the providers run over the same cables, but it's the sourcing that you're changing.