Just perhaps the "defrauding the treasury" lobby are taking a little too narrow a view of this.
Knowing many of the people that are lobbying for benefits to promote modal change they would not really care about the few that don't buy and use the bikes in the spirit the benefit is provided.
The wider socio economic benefits outweigh the possibility of a few cyclists taking the pee. More bikes and more visibility of it as a healthy and acceptable way of traveling whether commuting or leisure is a good thing and the cost/benefit to public expenditure (or loss of tax revenue) is a very hard thing to pin down with any accuracy.
Personally I'd be happy that a bit more of the general tax pot goes on promotion of cycling than into paying for inflated surgeon salaries to treat the ever increasing number of obese car drivers that wouldn't exercise if it was to save their life.
I have a load of bikes and have a bike to work scheme but have not taken it up. I have some colleagues who have bought bikes on the scheme and use their bikes in teh spirit of the benefit and those that don't. I don't begrudge any of them because collectively they are reducing the number of journeys made by car.