Green River - the safety bar would keep the top from "crashing" down on occupants if the cables should break as long as it is engaged. An easy test you can do is to try to lower the top without pulling the safety bar release. When you push the switch to lower the top, it removes all lifting force on ALL the cables, which simulates a simultaneous failure of all the cables. The top will not come down. If you do this, don't leave the top resting on the bar, push the lift switch up to raise the top off the safety bar and put the support of the top back on the cables.
Edit: Another forum member, english775, just pointed out to me that what I've printed above may be totally wrong. He says the safety bar simply keeps the hydraulic ram from moving and that it in no way "supports" the top. If this is correct, then the bar WOULD NOT keep the top from coming down if all the cables broke. So, I'm going to have to do a bit of further research.
However, I'm still not convinced the top would "crash down" if a cable broke. It might if ALL cables broke simultaneously, but if they failed one at a time, the top would sag on the corner that was not supported and it would tend to bind in the tracks, which would stop it from lowering. Indeed, this has been the experience of members who have had cable failure. The top binds and it will not move until the low corner can be lifted to match the others.
In any case, there is no instance reported that I know of where the top has "fallen" dangerously due to cable failure.
I'm not saying using 2x4s to support the top when it is up is a bad idea. Several members do this if the top is going to be up for a long time to take the strain off the cables.
The problem with having a powered jack on each corner is that you need some way to regulate the lifting speed of them all so that the corners are lifted at the same rate. If one moves faster than the others, the top will "bind" as it goes up and lifting will stop.
- Jack