TarheelsRule:
It isn't a tournament rule it is a rule of golf. You must play the same ball from tee to green, you are allowed to replace that ball after the hole is completed.
This is what I said, except that a player may take another ball (and another brand) through one hole if allowed by the rules, e.g. with a ball lost during play or damaged.
TarheelsRule:
The one ball rule is in effect for any golfer who takes his game seriously. Many tournaments, especially 'scrambles' in the states for example do not conform to the rules of golf and allow excess clubs, preferred lies, different balls, etc.
Excuse me, now you mix up quite a few things.
"Serious golfers" will use one type of ball on their own will and not enforced by written rules. IMO, the upper 5 % of amateurs may take notable profit from such habit.
Tourneys allowing deviations from the Rules, as excess clubs, aren't golf, strictly speaking, it's just a similar pastime.
Preferred lies are frequent on the Tour, more often than in amateur tourneys IMO.
Different balls are not a subject of allowance but of restriction by the One Ball Condition/Rule, as named in Appendix 1, Part C of the Rules.
And if you don't believe me, read this (special regard to the 1st sentence).
TarheelsRule:
In any USGA event this rule is in effect. Every state golf association tournament that I have played in has this same rule in effect. The PGA has a more strict rule in which the one ball rule follows from first tee to final putt, you are required to use the same exact brand and type of ball. You can substitute a new ball when your ball becomes unplayable during a hole and after you finish any hole.
This is what I called high-level tournaments. Maybe the practice is different in the USA, but over here I've not seen it in any tourney organised by a golf club in which I participated (low level).
TarheelsRule:
Most serious golfers play the same ball at least during a single round. In scrambles or tournaments of those types, players will often substitute a distance ball for their tee shots and switch to a spin ball for approach shots.
Voluntary substitution of a ball is against the Rules - the players are either subject to penalty strokes or DQ, or they don't play golf.