Mast retro kit

Without the sleeve, masts occasionally permanently bend at the deck, mostly when sailed hard by heavier sailors. The sleeve is intended to strengthen rather than stiffen the mast to reduce the likelihood of this happening. All it does is move the point of maximum stress to about 8" above the deck (depending on how far in you push it). This decreases the maximum stress and provides approximately a 7 or 8 percent increase in the allowable load before the mast permanently bends.

A lot of people never have a problem without the sleeve. However, the sleeve is an improvement and bent masts are now a lot less common. Bent masts can be straightened and there is no loss of strength due to straightening,
 
Note that the sleeve was introduced when the builder spec'd out the same thickness tubing for the Sunfish and other boats they produced. Thus saving money on the metal and reduced inventory.
That's when the mast bending at the deck problem showed up and the sleeve was introduced.
 
All the previous comments are on target. In addition, salt water getting into the mast, leading to internal corrosion, may contribute to the problem.

With respect to sailing in waves, try to avoid being stopped by a wave when going/surfing at high speed downwind, because all the 'forward power/momentum' will be transferred to the mast/maststep.

But if all you do is sailing on flat water in relatively modest winds (say up to 20 mph), I personally wouldn't bother putting a sleeve in.
BTW, current masts are sleeved internally.
 
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