Does a typical "half cage" for DE/track day use provide any significant rollover protection? I suppose it's better than nothing but if you are not using harnesses is it worth the investment purely for reason of safety? I am shopping for a street/track GT4 and some track-prepped cars come w/ them but most don't - how much value should I give to having one? .

A roll bar provides rollover protection. A harness bar provides a mounting point for harnesses and no rollover protection. Roll bars are most effective when installed properly and that usually means spreading the mounting load with steel plates to reduce the chance of punch through.

Yes, I mean like Cantrell, CMS, BBi, GMG, RSS, etc. Most do not go to the floor.

The half cage is part of the system. The issue is once u install buckets and a 6 point belt u can longer fold into the center of the car in a roll over so the cage reinforces the b pillar to keep the roof from crushing your head. Its important to have one with a decent design and point loads on the structural areas of the chassis.

"Its important to have one with a decent design and point loads on the structural areas of the chassis."

Are the ones that do not go to the floor effective? My scenario is that you are not using a harness, so is that type of cage/bar a good thing to have anyway? (To me a cage is something that envelops the driver, which is not what I'm talking about.)

If u r using a bucket then u should have a harness and if have a harness u should have a rear hoop as a minimum. If u just do 1 or 2 of the 3 u increase your risk instead of lowering it. Rear sections that mount to seat belt mounting points r still bolted to a structural area of the chassis so they do offer a good level of protection

if u r using factory seats and belts the facrory crumple zones r adaquit.