Mactracker provides CPU benchmarks for the majority of the Mac models, including the variants within the same model line, where the CPU speed differs. Since the list of Apple models is finite (large, but finite) and the specs are consistent between models, I would think there would be little variance in benchmark results for a specific model, once a particular model (for example, a "iMac10,1") is tested and the results posted, whether posted by Mactracker or just other users with the same model.
So if I own a iMac10,1 at 3.06GHz for example, why should I or should I not expect that my benchmark result would be significantly different from that listed in Mactracker, for the same iMac10,1 @ 3.06GHz?
If the answer is yes, it could be significantly (+/- 5%) different, then I would say, why? if the answer is no, i should expect pretty much the same result, then running the same benchmark when test results for a model have already been posted, seems redundant?
If one suspected there was a problem with their system, yes, running a benchmark might help troubleshoot a problem, but if it's just to gauge and compare between models, I would think the results posted in Mactracker are sufficient for comparison.
BTW, I'm not advocating that you shouldn't use Geekbench. I can see a use for memory benchmarking, as people do buy third party memory and would want to compare brands or the impact of more/less memory.