Which JAR?
JAR-23 has achieved unified safety standards for medium sized aircraft through most of the world, vastly reduced certification costs and much greater understanding of the certification system.
JAR-FCL has caused massive complication around Europe, offshoot national licenses in several countries, and huge cost and complexity to all concerned.
JAR-22 for the first time ever unified the standards for gliders, allowing sale of such aircraft across (most) national borders within Europe without massive and prohibitive re-certification costs.
JAR-VLA has provided a good source of reference and common ground for certifying non-aerobatic light aircraft in most of the world. Unfortunately it hasn't, due to the bureaucratic process, been amended for 12 years - so in effect that's 12 years in which ongoing safety lessons have been prevented from being ploughed back into the certification process.
JAR-25 meant that Airbus and Boeing could sell into each others markets and eliminated lots of blatantly protectionist measures that previously did nothing but damage.
JAR-145 introduced management systems which simultaneously sorted out many bad management practices in large maintenance organisations, and shut down many small organisations because they couldn't afford to function any more.
You pays your money, you takes your choice. But you could never say it was irrelevant anyway !
G