https://bitcointalksearch.org/topic/m.2712210 and the posts before that
DrGregMulhauser's answer in the linked thread is as eloquent as it gets.
I haven't bothered reading the filling, but I would not discount the inclination of retail investors to buy a fad without understanding it. As Zangelbert points out, institutional investors could easily kick up the price, attracting attention, then reap the reward of a saturated order book.
For the Twinklevii, fees are collected either way, so the ETF being proposed as an attack vector is ambiguous. Shares of the ETF would take on sort of a life of their own in the sense that they are largely separate from the actual bitcoins they represent.
While the fund can then be wielded as a leveraged tool, there is no way to stop the flow of wealth - it can only be accelerated or delayed. In other words: if Bitcoin usage continues to demonstrate benefits and garner new users, all the suppressing power of the ETF would only serve to allow more entrants and wider adoption at a faster rate.
I think it's more of an effort to diversify revenue-generating ventures. When a business is self-sustaining, starting another is a prudent step; the twins now hold a huge sum of bitcoins and may soon be pulling a commensurately large flow of fiat, and flow of funds is as important as reserve.
The real asset is all that matters in the end, though. Bitcoin will thrive or fail based on its own merits.