I don't want to dump on Musk, but from a shareholder perspective this stinks to high heaven. He may "recuse" himself from the vote, but just negotiating the deal is enough to make it likely to pass. It's pretty easy to induce the firms that make shareholder recommendations to approve even the worst possible deals (i.e. HP-Compaq) and that means a ton of pension funds/index funds will be forced to vote for it.
There are zero synergies here, unless you count more ways to obfuscate the financials of two money losing companies as one. Thats what deals like this typically look like and it's the reason someone made a Trump reference. Trump specializes in putting together whacky deals like this to keep his awful businesses out of bankruptcy and continuing to pay cash into his pockets.
Now I don't think Elon is lining his pockets here, he's clearly a builder who wants to create amazing things. But he's also thin skinned about criticism and not above gaming the system to his advantage, and it sure looks like he's going to use the money he's raising for Tesla to keep Solar City afloat longer. Which is pretty scary given Tesla is still years away from cash flow positive on any long term basis.