I think it's a good idea, in so much as it allows you to add some additional stiffness to the cage. Is it a bad idea? Maybe only in the fact that it restricts moving around inside the car a bit.
See the thread in General Discussions titled "Notes from presentation by Tom Gideon, GM Race Satefy Eng." by 924guy.
Long story short, it's probably a good idea if you can install the bar in such a manner that you don't slam into it during a crash. Also, make sure it is not in the way if you need to bail in a hurry.
A good alternative would be cross bracing the cage as much as possible, but that could a lot of weight.
They're bad if you're used to not having one and then try to give a point by to the pass side.
I about broke my hand the first time I pointed someone by! (which was probably you at GIR in March
)
I've seen quite a few cages where the Petty bar scared me it was so close to the drivers head. I guess some builders think chassis first, safety second. I toyed with placement on mine for quite some time since I plan on having an extra seat available for driver ed. days and wanted to give as much clearance as possible for both.