Us outsiders tend to think that the marketing people will influence the technical decisions (e.g., comments like "Peugeot will introduce road cars with HDi hybrid engines"), but in the end it is up to the engineers.
Weight is clearly an important factor. The Porsche 997 GT3 R hybrid system weighs a hefty 125 kg and the system of the Corsa Ginetta-Zytek 09HS 85 kg. However, the system of the Peugeot 908HY weighted 65 kg, but because some other components (e.g., starter and alternator) can be removed the car only weighted 45 kg more than the regular 908; Peugeot hoped that they could get it down to 35 kg (see
here).
According to simulations by AMR/Prodrive the old hybrid rules with 1000 kJ energy storage (instead of the 500 kJ announced for 2011) are good for 3 sec in Le Mans. So for manufacturers it is definitely worth the investments.