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.

Well, I'll play devils advocate.

1. A boat should know if she has broken a rule.
2. A boat that has broken a rule is required to take a penalty even if there is no hail or flag.
3. It therefore follows that to only take a penalty if and after the protest notification requirements have been met is at the least verging on unsportsmanlike conduct.
4. Given a really strict and literal interpretation of 44.2 it could be held that the time interval for hearing a hail and seeing a flag exceeds the delay permissable for starting to get clear of other boats, and therefore makes the alternate penalty invalid.

My own view is less extreme. I find it unacceptable that a failure to follow procedure precisely on the part of the injured party should constitute a get-out-of-gaol-free for the guilty, but it could justify a PC imposing a lesser penalty than disqualification.