Hi Alistu,
i completely agree with you. The reason that external modulated lasers are used is to overcome the power fluctuations and the high end chirp caused by the direct modulated lasers because of their ON-OFF operation especially when using RZ type of modulation scheme. It also limits the maximum transmitted bit rate of our systems. Undoubtedly, as you said externally modulated lasers have better performance. But is has been observed that for long reach networks, the performance is better for DMLs rather than EMLs.
regards