I don’t care if someone pays $5 or $50 a plate - you invite according to what the person means to you, not according to whether or not they might stiff you on a meal.

I would never NOT invite someone who I felt was important to me just because there was a chance they may not stay for the entire meal or have to leave early - what an insult to that person!
Trust me - I know what it costs to put on a wedding. Been there done that - twice.
Cutting down a guest list for budget is one thing - but choosing one person who is important but might not stay, over another person who is a sure thing to be there till dawn, is not the way to choose a guest list. If the money is THAT tight, and that important, then the couple is way over spending on the event and not keeping their priorities in order.
~Liza