Surely it's obvious that by far the most important reason why multiple units are in favour is cost. A new multiple unit compared to a new loco-hauled train would almost certainly fare less well in terms of passenger ambience, what with noisy underfloor traction motors/diesel engines, fixed formations resulting in lack of versatility in times of high demand, lack of versatility when it comes to different (non electrified, say) routes*, and noiser air-con (the reason units' aircon systems are noiser than coaching stock - even 30 year old Mk3s is that the power units take up the underfloor space which would otherwise be used for aircon, meaning that they are in the carriage roof, and consequently intrude more on the passenger compartment).
But anyway, I digress. Units are cheaper - they are charged less per mile by Network Rail**, they need less staff to operate, they require less extensive infrastructure (for running round), etc. If loco hauled stock were cheaper, I'd expect it to dominate.
*Thogh the French AGC has got round this, I find it extremely unlikely that it will work in the supposed high-speed IEP.
**Even more annoyingly (unless this has recently changed), Network Rail charge more for electric locomotives than for diesels - presumably because they cause 'wear and tear' to the overhead wires. While this makes sense, it also discourages freight operators from investing in electric locomotives!