I don't know where you get your information, but seats are designed NOT to break. If that were not the case, in a high impact rear collision, the seats would collapse backwards which is VERY bad. Pdf file of the DOT testing/standard requirements for seats in passenger vehicles (excluding buses):
http://www.gpo.gov/fdsys/pkg/CFR-2008-title49-vol6/pdf/CFR-2008-title49-vol6-sec571-207.pdf
What about a Porsche GT3 that comes STOCK with racing seats? Those are taken to the track AND daily driven. One could argue the fact that they are enhanced structurally for rollover protection, but I don't think you can test for EVERY possible rollover scenario.
Also, aren't rollovers reported less than 3% of all crashes? Even less is the amount where the roof totally collapses. Of the hundreds of rollover pictures I have seen (both on road and at the track) very few are ones where the roof completely pancakes. A lot of them you could barely tell they rolled over as there is only damage to the corner of the roof and windshield.
I am going to agree with you about the harness bar and 5 point belts for the street though. The bar is always in the way and the belts keep you glued to the seat where you need at least some movement to maneuver through traffic. Without being able to look over your shoulder easy for blind spots there is a high risk of making a bad judgement using mirrors only which would involve in a collision of some sort. Also, in the rare event that the roof fully collapses and the seat does break, you are right, the bar would stop this from happening. Thus why I said it would be better to have a roll cage. I don't recommend them for the street/daily driven though as they aren't really designed for your body bouncing around in side impact collisions found on the street. Should be wearing a helmet with a roll cage anyway, to save your melon from smacking the bar when bouncing around.
I guess I should make my point clear as I have jumped around a lot. I fully recommend that if you are going to get a racing seat, get a high name brand and not a cheap brand or a replica. I also don't recommend a harness bar and 5 point harness for the street as they are designed and tested for the track NOT everyday driving. I recommend a roll cage if you are going with racing seat AND 5 point, but for the track only and always wear a helmet. Roll cage is a recipe for disaster in a daily driver.