Problem with solid core cable, at least in my mind, particularly at large gauges would be getting a connector on it.

I prefer banana plugs for ease of connection and for giving a good mechanical connection and you'd have to either solder a banana to solid core or some how mechanically crimp it...not something I'd want to do on my own, whereas with multistrand you can use twist connect connectors.

Also you could argue that on the connection points multistrand gives you more contact area than a solid core cable would.