Holding Frank over until the ATFR makes sense if you're coming from the perspective of Ali does not get engaged to Roberto or pick Roberto in any way, shape or form. The angle they would hit if it's a no pick is either Frank was it for her on this season, or he was the reason she couldn't move forward with either of the F2. Either way, he'd be at the center of it all. Thus it makes sense for him to be there and not at the MTA.
If Roberto and Ali are engaged, Frank's appearance at the ATFR makes no sense, IMO.