A stiffer rear will end up unloading one of the wheels. With the 135i lack of a limited slip differential that means you end up spinning the wheel that has little load on it so you can't get power down coming out of turns. Once people upgrade to a limited slip diff they will consider bigger rear bars.
The reason it is believed that the front anti sway bar helps is that by keeping the front flatter the tires are not rolled over as far on in turns and there is less positive camber added allowing a better contact patch on the front tires which reduces understeer. In theory.
2011 BMW 135i DCT M Sport Coral Red Tint, Apex ARC8, 235/275 Star Specs, Ohlins Coilovers w/ Swift Springs, M3 front bits, Vorshlag