There is no purpose in setting up the fund if they don't intend to sell the shares. They don't have to sell all of their shares, but they have an incentive to sell a large portion.
Sure there is. Administration fees. They could setup a fund completely independent of their personal holdings and buy bitcoins on the open market. They could put their funds in the ETF, and buy/sell for other people via the ETF without touching their own shares at all. They'll make a profit on admin fees regardless of whether they sell their personal stash.
Read the link.
"Winklevoss have filed to float their stash of Bitcoins on a conventional stock exchange."