It's partly that branded stuff will last longer, and partly that they will also do spares for a minimum of like 10 years after you buy them. It may sound silly, but if in 5 years time one of the cartridges goes in one of the bath taps you bought from x shop goes and you can't get a spare then you need to buy whole new taps and fit them. Which is usually a pain with the bath in situ.
As Gilly said, you can get Bristan and other makes in the B&Qs etc, but just hunt around for a decent price.