Their not 'training bikes' their still normal bikes, just smaller, not everyone is comfortable with a stock bike, for those bigger people they usually start out on a stock bike, so no, mod bikes aren't just starting out bikes. Also, some stock bikes can also be fitted with a small seat too, not just mods. It just depends on which type of bike your more comfortable with, usually it goes with how tall you are, so if your relatively short it would be a wise choice to pick a smaller bike (like a mod) and if your taller, a bigger bike (like a stock). Simple.