Justin Wolfers speaks

Legalizing wagering on which team wins or loses a particular game,
while banning all bets on immaterial outcomes like point spreads, would
destroy the market for illegal bookmakers and make sporting events less
corruptible by gamblers.

And:

Point shaving may be widespread enough to have occurred in around 1 percent of N.C.A.A. basketball games.

Here is the full argument, and thanks to Chris F. Masse for the pointer; Chris also points me to this new and possibly very important study (a senior thesis by Jonathan Gibbs, at times the link isn’t working) of point shaving in the NBA.