Keyboard lock can on some BB's depending on the OS level keep you from answering a call because all keys are locked.
If you do not use a device password then you would need to remove the phone and press the keyboard lock icon to do anything if you lock it. As aiharkness indicated it does leave it open to someone using the phone.

Lock the keyboard
If you do not set a BlackBerry device password, you can lock your keyboard to avoid making calls and pressing keys accidentally.
On the Home screen, click the Keyboard Lock icon