In team racing we have the redflag, in match racing the y.
A redflag generally means a protest, if seen as it should be you can take a penalty.
I like the flag, as a protest committeeit is easyif you gave flown it. 
To me optimists and other small singlehanded boats had developed simple ways of displaying flags.
I agree the size distinction is not justifiable.
All boats should display flags if it was my call.

Just a question on boats like Optimist or ILCA where do you display a red flag all the race long until you finish? 
Secund thing, imagine a TP52 race with a lot of noise onboard due to the fact of carbone structure, do you really think you could hear «  protest » ! 

The answer is definitive if you are a keeper of records. The flag used to be required by all boats until 3 or 4 or 5 ... rule books ago, when the Racing Rules Committee made the change. Their reasoning should be in their minutes, or the memories of the older IJs.

As to Didier's question: kodak film cannisters, under the boom. But we don't use film anymore ...

  Using a red flag remains important in youth sailing. It helps reduce unnecessary back-and-forth on the water and clarifies when a competitor is actually making a protest. Out on the course we often hear things like “circles” or “do your doughnuts,” while the word “protest” is rarely used. Hoisting a red flag and stating “protest” serves two key purposes:

Verbal exchanges can easily be missed in the heat of competition or amid the noise and conditions of an incident—especially when contact occurs, such as masts touching or boats colliding. The red flag reduces ambiguity and ensures the process is understood by everyone involved.