You will probably spend a lot more money on specialized clothing and accessories than you will save in gas money.
When it rains you tend to get wet, but that's where the specialized clothing comes in.
Bikes are not too good in the snow and ice of winter.
You have limited cargo/people capacity. This could also be viewed as an advantage depending on who wants to ride with you.
Bug splats. You haven't lived until you catch a June bug in the throat at 50 MPH!