Alt-J started out as a quartet, but turned into a trio when bassist Gwil Sainsbury left after their first album and have gotten better and sold a lot more records. The band played NPR’s Tiny Desk Concerts in with both lineups, so you can see the difference: Old and New.
When Barry Andrews left XTC, it opened the door for superb guitar craftsman Dave Gregory to join the band — but it also rid them of a relatively unformed writer intent on recording his own material, which the two established writers (Colin and especially Andy) didn’t care for. Not to mention his keyboard style, while a perfect fit for an energetic New Wave band, would have stuck out like a sore thumb as the band’s songwriting matured.
It worked out fine for Barry though. Shriekback were pretty great.