This article, thinly masquerading as a scholarly work, is full of crap. It is a clear example of what happens when a non-programmer, non-technical person combines a word processor with a PDF creation tool.
The author, Ben Laurie, is actually a software engineer and cryptographer.
With 'truly valuable' I mean value that can be depended on, no matter what that value is. I certainly don't want to speculate about whether that value be higher or lower, but if it were higher, transaction fees would be worth more than now. But if I do a rough guess of the transaction fees (by inspecting a few blocks on the blockexplorer), they're now about 0.05 - 0.20 bitcoins per block.
That means, if exchange rates wouldn't change, a ROI of 0.2% of what it is now and with that an expected difficulty of 0.2% of what it is now. A not so huge investment is necessary for that.
I know, fees could rise, the exchange rates could rise, the number of transactions per block could rise.
If bitcoin is to be a successful currency, exchange rates and number of transactions will rise by orders of magnitude by the time coin generation per block has become negligible.
However, it's always safe to assume fees will be much less than total transaction value in a block, and therefore it's lucrative to calculate hashes of an forked block-chain with double-spent transactions.
Once again:
The cost of producing hashs is not a short term cost. It requires a long term investment in the hardware that produces them, so unless there's a way to double spend for hundreds of blocks without crashing the value of bitcoins, it would not be worth it. It would be more lucrative to just be honest.