It wouldn't be daft but it would be more expensive. If we assume that someone does a PPL(A) in minimum hours at £100/hour, that would cost £4,500. This will only exempt them from a maximum of 6 hours from PPL(H) i.e. 39 hours at £200. The remaining 6 hours in the helicopter would only cost £1,200.
I've never flown fixed wing so I can't comment whether the airmanship really did eat that much into my capacity. However, it takes an average student longer to complete a PPL(H) than a PPL(A). 60-70 is not uncommon.
Cheers
Whirls