The point about the extra weight is it consumes additional energy every time its accelerated over a straight electric versions which cost money but also potentially leads to unnecessary emissions in generation to provide that energy.
I appreciate that it's extra weight on the tracks (and extra fuel) but the alternatives also have negatives.
For example, if you are looking at the MML, do you:
a) order bi-modes for the partially electrified services (i.e. to Nottingham/ Sheffield)?
b) cut the Nottingham/ Sheffield services at Kettering (until the wires reach all the way to Sheffield)?
c) continue to run pure diesel trains from London to Nottingham,/ Sheffield until the wires reach all the way to Sheffield (thus continuing the air pollution at London)?
d) drags north of Kettering (so getting something like a 68 to haul a Sheffield - London EMU as far as Kettering, where it'd uncouple and haul the next northbound EMU to Sheffield)?
(I'm assuming that you have pure EMUs for the Corby services, and plan to eventually wire up the line to Sheffield in the medium/long term, but what do you do whilst we wait for the wires to complete the whole route?)
Everything has drawbacks/ costs - a "68" hauling the EMU is more damaging on the track than the relatively distributed weight of a bi-mode 810.
And the MML example is a lot simpler than the GWML/ ECML - given the complications of "branches" with infrequent services (Lincoln, Harrogate, Hull, Sunderland, Inverness, Penzance, Milford Haven etc) - how do you handle that? And the fact that bi-modes allow interworking, rather than a southbound Sunderland/ Lincoln service having to wait to form the next northbound Sunderland/ Lincoln service? Or do you trim the service pattern back to only electrified locations? Or have a 68 sat idle on Tyneside for long periods in between hauling a Sunderland service as far as Newcastle and then waiting for the next London - Sunderland service to haul it back to Wearside? (and the same for Lincoln services at Newark, Inverness services at Edinburgh etc)
In the context of that, the additional weight of bi-modes seems a necessary evil IMHO (not saying it's
perfect, just cheaper/better than the alternatives)
Coupling up a diesel (or other self-powered unit) at the limit of electrification (Leeds, Newark, Newbury, Edinburgh/ whatever) would save dragging the extra weight about where it's not needed. Used to be done every hour at Rickmansworth, Bournemouth, and other places very slickly. And is still done at Edinburgh six nights a week
Presumably a lot easier to do at Waverley in the middle of the night than trying to arrange at regular intervals during the whole day (e.g. if the Aberdeen/Inverness services were "dragged" north of Edinburgh then where does the loco go in between times - given that platforms etc have been rationalised since the 1980s - we don't have a lot of head shunts and middle roads these days)