Patch release procedures are covered under agreements with the various publishers. FWIW, Larian did say they made sure they would be able too release patches more quickly for FoV, if required, when negotiating those agreements.
Releasing a patch before FoV (and any associated changes to D2:ED) is done would necessitate another patch release when FoV was finished, to include subsequent changes. Publishers may be unwilling to go through multiple testing cycles, and Microsoft has limits on the size and number of patches for Xbox games.
I've not seen FoV being pushed as a fix for D2:ED problems, and it is common knowledge a patch will be released around the add-on's release to update D2:ED without requiring FoV.