Pre clearance would have been the obvious advantage for carriers such as AA UA over Heathrow. I cannot see it having any effect on TCX Norwegian pax etc as their client base is likely to book ex Manchester regardless of PC or not. Those punters will doubtless not have a clue.
Yes of course it helps pax from the Manchester catchment area BUT it should really have been a major catalyst to swing traffic to Manchester as thee only UK airport with the facility.
A truly golden opportunity.
It would be a bitter irony indeed if in the very year it gets the go ahead the client base for which it was really required evaporates.