A bastard sword is generally considered inferior to the Battlesword but it has its advantages, such as being usable with one hand or both, allowing you to do something with the other hand - such as throw a javelin - at the same time.
URW Game Encyclopedia description
The official URW in-game encyclopedia (accessed by the key) describes the bastard sword thusly:
Any hand-and-a-half sword 40-48" in length. They have a longer hilt than a broadsword, and their name derives from the fact that they may be used either one-handed or two-handed.