There's a small section of longitudinal seating at the end of some 321s too - I think it's just to occupy an unusual size space once occupied by a first class section though. I can't see any stock that doesn't already use longitudinal seats being converted to do so, it'd be too controversial. Fair play for the stock that already uses it though. I'd say up to 30 minutes on routes that are 100% loaded on a regular basis, longitudinal is the way to go. On routes where it's common that journeys are in excess of 40 minutes though, I think it's a bad idea, especially full longitudinal.
The c2c example came up, I doubt they'd ever do it (and don't think they should) as while it'd be fine going as far as Basildon or maybe Grays, the people commuting to Leigh-on-Sea and Southend would be sitting in uncomfortable seats, or more likely standing, for a considerable period of time. Except in special cases longitudinal seating is best left to 'rapid transit' rather than proper railway lines IMO.
