You don't negate the need for an SSD. Any HDD is still horrifically slow by comparison - if that weren't the case, replacing your PC HDD with an SSD wouldn't make much difference. Ithink the choice is between something like 24+ GBs RAM and a large HDD, or 16 GBs RAM and a smaller SSD, and the latter will provide overall a better user experience. I suppose there's a third option of HDD + more RAM + on-board flash for OS keeping that responsive, but then you're adding complexity and costs.
As I said before though, I think the moment you have a platform with a guaranteed SSD, devs will target it in ways they wouldn't target an HDD. As such, speed comparisons at the moment aren't much use. We're comparing HDD-designed performance on an HDD versus HDD-designed performance on an SSD. You could potentially load less to begin with because the streaming in of assets will be faster, or use better procedural content (create once on the fly but save for later use) with simultaneous streaming and writes to storage which would mess up the head movement and seek rates on HDD.