I find it bizarre that a supplier either get's pissed off or drops you as a customer if you SCAR them.
Just for the SCAR...likely not. But there's never only one part of an equation.
I think that sometimes folks writing (or considering writing) a SCAR forget that the supplier does their own math.
"The customer is always right" may work in retail, but not in commercial...especially when the customer makes up an insignificant amount of the bottom line. If the vendor makes less money on your business than the labor it would take handling the SCAR...the math gets really easy. Filing your SCAR under "unfounded" and no quoting you moving forward is a viable way of handling the situation.
I have customers I bend over backwards for to make them happy...and I have customers I wouldn't miss ... and I have non-customers that I simply don't want to deal with and thus do not do business with even when they ask to do business with me.
Every customer is in one of those buckets (or the "nice to have around, but wouldn't hurt us" bucket). Folks correctly bucket suppliers...but how often do we realize that we, as customers, are bucketed too...
Before writing a SCAR, especially to a sole supplier, consider carefully what bucket you are in as a customer...perhaps even ask them, though you should already know when it comes to sole suppliers.
As Ed said above...
We issued a SCAR for a product that was a sole supplier. They dropped us and well that product went away.
Consider the risk...there is no cookie cutter answer.