This argument has been done to death. As passengers I suspect much prefer an extra carriage or two to the dead space that a locomotive produces. And it's certainly less bonkers than continuing to sling long-distance diesel units under the wires, or going through the whole shenanigan of locomotive swapping.
TPE's recent order for new LHCS suggests there's plenty of life in the argument.
I don't understand the statement "prefer an extra carriage or two to the dead space that a locomotive produces". The incremental space taken up by a locomotive compared to a driving trailer (with built in Sexy front end, cab,crumple zone etc.) wouldn't provide for one extra carriage, never mind two.
I can appreciate that this marginal increase might be valuable in some cases but I cant see this for the ECML. The new 9 car class 800/1 are quoted at 242m whereas an 1c225 set is some 249m and seems to fit in the existing platforms with space to spare. I would suggest you could have bought LHCS+Loco with a similar passenger carrying capacity to the class 800 that would have fit the platforms and given the passengers a much better journey.
This morning I travelled in a class 322 MS. It provided a clear reminder that the noise from the traction kit on acceleration can be very intrusive.
Similarly " the whole shenanigan of locomotive swapping" seems nonsensical. Whatever's difficult about loco swapping? The time taken need hardly be much more than the time realistically needed for passengers to join/alight at the likes of Waverley.
The fools who authorised the class 800's are the same fools who authorised the associated GWML electrification and we now know how much thought they had given to that.