For starters, it is cheaper and simpler than a regular Catenary+Contact wire configuration seen on IR's main lines. It is also cheaper to maintain and construct.
The speed is restricted. Normally such lines are good up to 80kmph. But with the mixed track conditions here (freight + passenger), the speed limit could be lower as well. The restriction will be only on electric locos as the diesel locos do not depend on the OHE.
Since the wire contracts and expands with temperature, and...
more... can also oscillate due to the upward pressure of the pantograph, a good contact cannot be maintained between the panto and the wire at higher speeds. This reduces the amount of currrent drawn and subsequently, is not suitable for high speed runs. In a catenary system, this problem is solved by having a second wire above the ocntact wire, which holds the contact wire in a relatively stable position.