Free formats are not enough.
Proprietary packages that export Free formats may do so poorly, leaving the output corrupt or incomplete. The code cannot be fixed except by the authors, who may be unable or unwilling to do so.
Proprietary packages that export Free formats may displace Free packages, so that when the Proprietary package stops supporting the Free format, artists are left homeless.
Proprietary packages that export Free formats may “embrace and extend” those formats.
Proprietary packages that export Free formats may not work on a given platform (free or proprietary). Their users cannot port those packages to their platform of choice, and the proprietary authors may be unable or unwilling to do the porting.
Given all this, a Free package with its own format is more free, and better for artists, than a proprietary package that exports free formats.