In my humble opinion, after the advent of bitcoin, there is no longer a reason for a client to do proof-of-work. Bitcoin basically outsources proof-of-work to miners, and presents you with a token that represents proof-of-work. As money, bitcoin proves - just as proof-of-work - that you've worked for it, there is no such thing as free money. The price of a unit of proof-of-work varies wildly, depending on what hardware you have. The price of money is the same for everyone, or at least varies much less.

The problem with proof-of-work is that attackers can invest in powerful hardware - as your spec notes - and have an advantage over other nodes. When paying in bitcoin, as a spam prevention mechanism, no one has an advantage over others. Everyone pays the current market price for bitcoins, and even the miners - who generate bitcoins, and so don't pay the market price - can sell their coins at the market price, so it still comes down to "do I want to use these bitcoins to spam this network, or do I want to sell the coins for $575 per piece?".