You seem to be far from alone in wanting this as Out-Of-The-Box functionality. There are at least three ArcGIS Ideas that I would recommend you add your name to:
In the meantime if someone is inspired to write a BookMarks To Feature Class tool, then I am sure its output would be well used as an Index Feature Class for Data Driven Pages.
I ended up doing this as a Training exercise based heavily on some Sample Code the ArcGIS Online Help for ListBookmarks (arcpy.mapping).
# The map with the bookmarks
mxd = arcpy.mapping.MapDocument(r"C:\polygeo\Maps\Bookmarks.mxd")
# Make sure that Training.gdb exists
fileGDBFolder = (r"C:\polygeo")
fileGDBName = ("Training.gdb")
fileGDB = fileGDBFolder + "\\" + fileGDBName
if not arcpy.Exists(fileGDB):
# The output feature class to be created -
# This feature class will store the bookmarks as features
fcName = "Bookmarks"
outFC = fileGDB + "\\" + fcName
# Create new feature class and add a "Name" field to store the
# bookmark name. Provide it with the same Spatial reference as
# the data frame in which the bookmarks of the map are stored
"Geocentric Datum of Australia 1994"))
arcpy.AddField_management(outFC, "Name", "TEXT", "", "", 50)
# Use arcpy.mapping.ListBookmarks to read bookmark corners and names,
# then arcpy.da.InsertCursor to write arrays of Point geometries from
# that can be written as Polygon geometries to the Shape field of the
# new feature class (with their names).
cur = arcpy.da.InsertCursor(outFC, ["SHAPE@", "Name"])
array = arcpy.Array()
for bkmk in arcpy.mapping.ListBookmarks(mxd):
# To close the polygon, add the first point again
print "Bookmarks feature class has been created in " + fileGDB