To sum up..
1. the point of a lighter weight bike (aka race bike) is to allow a rider to complete a set distance in the shortest amount of time possible, using the least amount of effort possible.
2. a lighter vs heavier weight bike should not have much impact on the quality of a structured training workout
3. a heavier weight bike can yield its user a tiny bit more exercise, if that rider's rides are of a set distance. Someone in a race however, isn't looking for more exercise, they want to complete the race distance as quickly as possible (leaving out the goals of domestiques).