The plastic shell is really only there to hold the foam together and help the helmet slide across the pavement if you go down. The foam is really what keeps your head from taking the direct impact of the blow.
I would hate to give up the ventilation of a bike helmet, but a BMX or skate helmet would be fine. A nice compromise could be the Bell Metro line (without the visor). It has a rounded shape reminiscent of skate helmets but considerably better ventilation.