I think the main reason the electricity companies discourage this is the hazard it presents. The 'collector' is basically a current transformer, which means:

1. If the outputs of the 'collector' go open circuit (because the rectifier/capacitor/whatever dies) the voltage at those terminals is going to rise theoretically towards infinity - insulation breakdown and discharge will occur which is likely to result in a fire.

2. The current generated by the 'collector' is going to be proportional to the transmission line current; if a ground fault occurs on the line the current in the collector winding is going to spike up to some silly level; that will probably result destruction of whatever is connected to the collector outputs and subsequently a fire.