With all sorts of racing in GT5, you can race online

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Some people just prefer time trialing, even with drag racing. I'll admit, in normal circuit racing, it's a less-broad skill-set than what's required in a race against opponents, since one would need to know how to pass and interact with others, but in drag racing, where you shouldn't be interacting with the other car in any real way, the only thing missing in offline versus online is the reaction time. No doubt important, I'd agree on that, but I wouldn't discredit the tuning that goes hand in hand with it.
Chalk it up to diff'rent strokes. I don't understand the obsessive nature of some of the marketplace regulars, but that's what they want to do with their GT5.
Besides, online and offline physics have finally been fixed. Or so I've heard...