I use synthetic in my bike. Why? I honestly can't give an exact, perfect, be all to end all, explanation. It's personally simply a preference. I look at the RPM, temp, etc my motorcycle sees. It's not severe duty, by any stretch of the imagination, but (as mentioned) synthetic oil is more consistant viscosity wise, more resistance to break down, etc etc.
Here is a little info I grabbed awhile back...
Using Synthetic Oil?