A rear bar works especially well on tight twisty circuits like hillclimbs where a little bit of oversteer allows you to be on the throttle early to power through long or tightening corners. It also means the driver has to be ready with the steering to compensate for the oversteer and use the engine to pull out of it. If you want to reduce understeer yet retain rear end grip you can use a rear bar but also play with the tyre pressures, rear end camber and toe-in settings, they have just as much an effect on handling as a rear bar does. A rear bar by itself may actually slow you down.

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Curly
'64 Cooper 'S' (Group Nb spec)
'67 Mini Marcos GT www.mm7056.wordpress.com

A rear bar making your handling worse is one of the more bizarre things I've read here. However of course a poorly set up rear bar will kill your handling. Don't blame the bar though. Another thing to consider here is the difference between front and rear wheel drive. Don't go reading a write up on roll bars for RWD then go trying to apply it to a mini....or you'll come away thinking rear bars make you slower......