The trade off is a little higher fuel comsumption, since some fuel will escape before the piston closes the exhaust port.

The idea is to facilitate the breathing and to increase the intake of the fuel-air mix into the crankcase.

For some two-stroke engines, just for experimentation, I have polished the intake of the carburetor, have aligned the hole of the throttle and the carburetor throat, have aligned the discharge of the spray bar exactly to the center of the ventury, have added a scoop to redirect the blast of the propeller (in an angle) into the carburetor.
All with good but not spectacular results.

Better off is a tuned pipe, and a correctly tuned engine.

A good reading about tunning can be found here:

http://www.flyrc.com/articles/tune_your_engine_1.shtml