The problem is that you can't just "tag on" a couple of extra coaches to a LHCS, where it is the work of but a moment to add (or remove) additional coaches from a train formed with MU stock. To add coaches to LHCS would take some time and require both stabling and shunting facilities at each location, all of which costs money. The only services that regularly do this are the sleepers, but that's only because they have all night to get to their destination.
The other LHCS myth that stands busting is what happens when you get a defective vehicle. Although, as with adding or removing vehicles, it is theoretically possible to remove a defective vehicle, you will still need the self-same stabling and shunting facilities and incur the same time penalties to achieve this. Therefore, as LHCS is generally now formed up into semi-permanent sets, the entire train would require removal from service.
In operational terms, virtually all surviving LHCS services and all HST services effectively use the same principles as MU operation. You have a fixed or semi-permanent formation of vehicles with a traction unit and a cab at each end. Within certain parameters at least (e.g. removal and changing of a loco), you just treat the entire train as a single unit.
O L Leigh