Positive effects:
-Lowers noise level;
-keeps noise level within reasonable limits, so that dirt biking is not banned everywhere;
-helps engine tuning for a smooth, wide power-band.
Negative effects:
-adds some weight.
Positive effects greatly outweigh negative effects. This is why dirt-bikes come with silencers.
Back in the old days an expansion chamber exhaust was the only 'way to go'.
Yes they were noisy but they provided the best bet for performance in the riding field.
The newer 2-strokes are designed to use the silencer as an expansion chamber of sorts, causing back pressure while quieting the sound.