Aside from the fact that the towers are probably not stressed for that type of use I doubt the voltage output of the wind generators matches the high voltages of the transmission lines. Perhaps supplemental transformers might remedy this and also adapt the irregularity of wind output to the required steady tension of the regular electricity normally transmitted.

Those lines require large substations to house the step-up transformers and other equipment necessary to provide the ultra-high voltage (110,000 volts to 1,200,000 volts) that drives the transmission lines. So, they could not be used to fuel the grid, just local demand. But, that adds complication as they have to account for all these little rotors on all these little towers and install stations and more equippment to get it to homes.

They use helicopters to service those lines, because it's cheaper than the equipment necessary to do it from the ground. You move a cherry picker up to one of those lines, and you'll have quite the show of sparks as the transmission line shorts to ground. You can't use a helicopter to service something if you've have a whirling rotor on the line. So, they'd have to add more complexity again, with remote stops and other control equipment.

Additionally, everything on that tower is electrically shielded. There's nothing but power lines on it for a reason - anything else up there makes it expodentially more dangerous. They don't even put cellular antennas or repeater stations on those things.

All of these problems could be engineered around and overcome. But at a high cost, with great difficulty and added complexity.

The towers are not designed to withstand the weight of a wind turbine. Im sure that they can be modified to do so but only at a high cost.Other than that, there are environmental issues as well. Wind turbines disrupt local low level winds, this can have a very negative effect on the local bird population, in some cases they can even cause the death of a large portion of said bird population.

If a turbine lost a blade it could cause a catastrophe. Also the maintenance people for the turbines would be working around very high voltage.