Primarily because of the lack of brake continuity through the train, but remember also that modern MUs have no buffing gear. A collision with the loco within the confines of the slack in the coupler due to a sudden brake application from 30mph could cause damage to both loco and MU. Having seen a Tightlock fitted EMU rescued by a conventional loco it's done by passing the end of the loco's shackle through the slot in the Tightlock's jaw and dropping the "mushroom" adapter through a vertical hole that runs through the end of the jaw to secure the shackle link in place. However, as the MU has no buffers the shackle can't be wound in to reduce movement meaning that there can be a fair bit of slack between loco and unit.
It's also worth pointing out that an unfitted freight would have had a brake van with the freight guard at the rear of the train which a loco + MU rescue formation would lack. Granted there would be a person on the MU to apply the parking brake in the event that it was needed, but the amount of brakeforce that provides can be very low given that it's only really intended to prevent the MU wandering off on it's own when it's own air supply is exhausted. I suspect that, if the Cl319 on which the somewhat heavier Cl769 is based on is similarly provisioned to the Cl317s I used to sign, the parking brake only applies on the lead bogies on the driving trailers and, if applied by operating the PBIC in the cab rather than exhausting the main air supply it would only apply on the bogie at the end of the train and not both. I was told when I was being trained that the spring operated parking brake is equivalent to a Step 1 brake application. Given that it would only be operating on 1 out of 8 bogies it would mean a 1/8th Step 1 brake equivalent which really isn't very much at all. It would take quite a while for a brake application that light to stop an MU from any appreciable speed.
