For on road use, the wider track of the wheels gives slightly better handling, especily if you've got lifted suspension. They allow you to fit wider tyres and keep the turning circle near standard but the extra stand-off allows more water than normal onto the front brake discs when it's wet, meaning reduced braking!!
I've had a set on Scorpion's alloy spacers fitted on the Disco for about 2 years now and not had any problems with wheel bearings, although I did replace all the brake discs just before fitting the spacers so the wheel bearings were all greased and correctly tightened.