Your point is well made and well taken. Most benchmarks are artificial and bear only limited relationship to how a particular game will play on a particular machine with a particular GPU and other components and set up in a particular way. Even how someone plays a game can have significant influence.
But there's no way to quantify all that, so they make benchmarks. Unfortunately, with too much emphasis on benchmarks, things like NVidia's tweaking cards specifically to max out the benchmarks at the expense of general usage occur.
It can get you high numbers and generate sales from those who worship benchmarks.
Its like focusing on the specs and deriving the notion that because a centrino machine can match some performance of a standard P4 with twice the clock speed, its a better machine. Clock speed is only one of many interelated issues. The main reason the centrino machines do perform well is their large L2 cache. If you gave a P4 a 1 MB L2 cache, it would smoke. Why they don't, I sure don't know. Its one of those questions like why have they been teaming the Athlon 64 with 64 MB ATI 9600s. Why not give it the full monty? Why, I sure don't know. Perhaps they have a lot of 64 MB ATI 9600s or the cost of a 128 MB would push the price above some marketing limit they believe the machine wouldn't sell well above. Or maybe somebody made up a bunch of mobile mobos that can only accept a 64 MB video system and they want to get rid of them before they do the right thing. What ever it is, its a safe bet there's some financial gain for someone in there somehow. Always is, always will be.
In short, the performance of any machine is the result of a lot of interelated features. Better than a comparison of cards and chips would be a guide on how to tweak them. If you up that, it will get you this, but cost you that. If you drop that, it will cost you this but gain you that. That's priceless information that takes more than just running a benchmark on a couple of different setups. Its also counterproductive to the usual policies and practices of most marketing departments. They want sound bites, quick sexy bits that are easy to convey and get people to buy, buy, buy.
Its like the obsession with FPS, you will notice those that really push higher and higher FPS generally have an interest in selling new graphics systems while those that question excessive FPS generally have nothing to gain. Does that make the arguments of one better or worse than the other, not necessarily but it does give one pause to reflect.