Thing is, I doubt many people will buy Steem to power up, when they can just buy SP directly through blocktrades.
So I highly doubt this is what gives Steem value.
I would think advantageous interest on SP holdings would suffice to attract people to buy SP - and I would maintain, even more so, than the less tangible value proposition of influencing user interactions on the platform.
When you buy SP at blocktrades you are buying steem and then transfering it to your vest account. So no, you can't buy SP without getting hold of steem first.
Science!