When users buy hardware, they don't necessarily but it with Linux in mind. Also, Nouveau is already way more advanced that the Nv driver it replaces. To give you an idea of a similar thing, think about Pidgin. Pretty much all the protocols it supports has been reverse engineered without any documentation. Ideally you would only be using things like Jabber/XMPP but in the real world but support for all those proprietary protocols in Pidgin has of course enormously benefited us. So has support for proprietary protocol in Samba and a proprietary documentation format in OpenOffice.org. Nouveau is very similar. The combined support of Intel and ATI with documentation and code along with the pressure of a reverse engineered driver getting more and more features will hopefully get Nvidia to change their minds. There are many good examples of this. Java going GPL is a recent case.
i always had the impression that reverse engineering HW is by far more difficult than reverse engineering SW