One word - Comfort (on inter city trips)
In fairness compared to other older ICRs the 22k sets are an improvement, but nowhere near as comfortable on a long trip as the mk3 and mk4 coaching sets. Aside from noise and vibration, the main difference is the hard SEATS which are very uncomfortable on a long trip. Fine for a commute but not for long rail journeys. As for the 2600, 2700, 2800 and 29000 (really 2900) they remind me of my time working behind the "iron curtain" many years ago. I understand the economics of ICRs, but UK, France and Germany can operate fixed rake intercity loco sets that don't need shunting nor run arounds at termini using mainly twin power cars at each end of the rake, or push pull. Same cost savings on labour, simpler track work at termini, fast turnaround. The 22k sets are just not comfortable enough for "Inter city" routes. The motorway is more comfortable. Nothing to do with emotional nostalgia for locos, just quiet coaches that are comfortable to sit in for 2 to 4 hours.
On trips to Cork and Kerry we only go Irish Rail if we can book on a mk4 set. Never again will I do a 3 hour rail journey on those 22000 seats.