IMHO you do not need a justification. If you think multiple links to the
same provider don't buy you what you need (e.g. if the ISP has severe
problems with its internal network multiple links do not buy you
anything. Same holds when your ISP goes south which still happens now
and then these days) go for real multihoming.
What is the prevailing wisdom now? At what point do you feel that it is
justified for a non-ISP to multihome to multiple providers?