If you're dealing with loose racks (wooden triangle) and tight pockets and opponent racks, the break at nine ball can be a disadvantage.
Back in the 1980s Pat Fleming did the statistics for matches in major tournaments on the pro tour. The percentage of "breaker wins" was under 50%. That means that not only was the break not an advantage, it was a disadvantage to the average player on the pro tour. Of course the break is somewhat different now, but if a player is under 700 and doesn't have a wired ball in the rack, have him break.