An open mag capacitor would make the spark very weak, and starting
would probably be impossible on such a mag. However, at higher RPMs
there might be enough spark to make the engine run. It'd raise hell
with the points in short order though.

I'm always surprised that there isn't more emphasis on what to expect
when doing a mag check at different speeds - like at idle which will
show bad points or that bad condenser. At cruise you could find a bad
coil or a harness problem - but you better do that near an airport
just in case. I wouldn't ever do a mag check at full power except on
the ground.

There are some recent ADs involving mag capacitors.