I think there's a large amount of cargo culting going on regarding the hash rate rather than useful analysis of threat models, attacker capabilities, and exactly what proof of work accomplishes.
I agree.
My guess is that we will end up with a very secure system with a modest amount of hashing in the future, because PoW hashing does three things:
1) Gives a steady 10-minute 'heartbeat' that limits how quickly new coins are produced
2) Makes it expensive to successfully double-spend confirmed transactions
3) Makes it expensive to censor transactions
The first becomes less important over time as the block subsidy halves.
I think we could do a lot to mitigate the second (see https://gist.github.com/gavinandresen/630d4a6c24ac6144482a for a partly-baked idea).
And I think the third might be mitigated naturally as we scale up and optimize the information sent across the network (there will be strong incentives to create "boring" blocks that don't include or exclude transactions everybody else is excluding or including).