That wouldn't solve the problem, they could just as well load more stuff from the disc to populate RAM (okay, maybe the load times would feel like forever)
The problem is actually making use of whatever is already there in the memory. The slow bus really cripples the entire system, making it very hard to imagine why they did not deal with such an obvious bottleneck...
Besides using what's in the memory, consider that you have to get data into memory from somewhere whether it be from the disk (horribly slow) or from the HDD/flash (also slow compared to memory), then you realize that having more memory even if it's not faster, will improve situations where you have larger datasets, even if it's not all "used" at the same time.
If you had 600MB's of data for a level, of which 300MB's is used at any given point, on WiiU the whole dataset could be held in memory, but on the otherhand on the Xbox360 you would have to do a transfer from the slow media to memory before being used (mid level loads)
So yeah, Wii U won't be able to use more of that memory at a given time due to bandwidth, but that doesn't mean there isn't an advantage of having more memory.
Because it's likely Microsoft's priorities have changed. In order to turn Durango into the set top box they wish, they need a lot more RAM. It'll be built from the ground with heavy set top box considerations, very unlike 360.
Don't forget that Microsoft might include the next gen Kinect with every box, it would make sense that they would just make that an OS level function, so developers would just use the API and it wouldn't take any resources away from the games allotment of ram/cpu.
Also, MS (smartly) limited themselves to 32MB's of ram with the Xbox360, which was great for developers, but was a PITA for themselves as they needed to squeeze all these changes they've done with the new iterations into that measly footprint. I would be that the dashboard would not have been so laggy if it could use more memory. They may want to "reserve" more for themselves this time around, for futurerpoofing.
Overall, it seems that Console OS's are getting more complex, offering more services than they did at the beginning of the previous generation. Though, if MS/Sony don't do true multi-tasking like the Wii U does (it has to to really make use of the secondary screen for the web browser and such), they might not need as much. It wouldn't surprise me if they nabbed 512-1024MB's for OS
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