When we built our robot in '00 once we grabbed the bar we couldn't let go. We had a "cam" that snapped in place once the dual hooks grabbed on effectively closing the hooks completely around the bar. We decided early on in the build to do this to prevent our robot from getting knocked off. If another robot hit us our bot would basically swing back and forth. If the other robot didn't back up quick enough our robot would swing back into theirs!
Also a latch on the arms was activated by a servo once we pulled ourselves up so that when the power was cut at the end of a match the arms wouldn't extend thereby dropping our robot back onto the platform and lossing points. We were also aware of the possibility of damage from other robots so all the electronics were kept towards the center of the robot as possible and all sides were shielded including a "skid plate" underneath. Our robot did recieve "battle scars" but nothing serious. The downside to grabbing the bar and not able to let go is you have to do all your "driving around" in the match before you would consider going for the bar. That could have changed if we added say another servo or something to release the "cam" but our stategy was to keep other robots off the bar (except our alliance).